How the Body Ages
TL;DR. Ageing is the gradual loss of the reserve capacity described in Chapter 1. Most organ systems lose roughly 1 percent of function a year from around the age of 30, which is invisible for decades because the reserve is so large, and then becomes suddenly visible when a system drops below the threshold daily life requires. That is why so much of ageing feels like it happens abruptly: nothing changed suddenly except which side of a line you were on. The rates of decline differ enormously between people, and a substantial share of the difference is not fixed biology but accumulated disuse, disease, and exposure. Peak capacity in early adulthood and rate of decline afterwards are both partly under your control, and between them they determine how many years you spend independent.
Key takeaways
- Reserve, not function, is what you lose first. A 70-year-old's kidneys work fine at rest and cannot respond to a challenge the way a 30-year-old's can.
- Most systems decline at roughly 1 percent a year after 30, but aerobic capacity falls faster (about 10 percent a decade) and muscle power faster still.
- Frailty is a distinct clinical state, not just old age, and it is partly reversible with resistance training and protein.
- Healthspan is not tracking lifespan. People are living longer with more years of disability, which makes compressing that period the actual goal.
- The biggest single reversible contributor to functional decline is disuse. Masters athlete data suggest a substantial fraction of what is attributed to ageing is deconditioning.
- Ageing research is genuinely progressing and is far ahead of its evidence in commercial form. Nothing sold today is demonstrated to extend healthy human lifespan.
What ageing actually is
In short: An accumulation of molecular damage that outpaces repair, producing loss of reserve rather than sudden failure.
Ageing is not a programme that runs on a schedule. It is what happens when damage accumulates faster than maintenance clears it, across many independent mechanisms at once. Biologists have catalogued these into a set of hallmarks of ageing:
| Hallmark | What it means | Where it shows up in this book |
|---|---|---|
| Genomic instability | DNA damage accumulates in every cell | Cancer risk rises steeply with age (Chapter 24) |
| Telomere attrition | The chromosome end-caps shorten with each division | Limits how many times tissue can renew (Chapter 3) |
| Epigenetic alterations | The pattern of which genes are switched on drifts | The basis of "epigenetic clocks" that estimate biological age |
| Loss of proteostasis | Misfolded proteins accumulate faster than they are cleared | Alzheimer's, Parkinson's, amyloidosis |
| Mitochondrial dysfunction | Power stations become less efficient and leak more damaging by-products | Fatigue, reduced aerobic capacity |
| Cellular senescence | Cells stop dividing and secrete inflammatory signals | Chronic low-grade inflammation, the target of senolytic drugs |
| Stem cell exhaustion | Renewal pools deplete | Slower healing, thinner skin, weaker immunity |
| Altered intercellular communication | Persistent low-grade inflammation, sometimes called inflammaging | The connective thread through cardiovascular disease, diabetes, and dementia |
| Chronic inflammation and dysbiosis | Added in the 2023 update |
None of these is the cause of ageing. They interact, and no single one has been shown to be the master switch, which is why interventions targeting one at a time have so far produced modest results.
The rate of decline, system by system
In short: Roughly 1 percent a year from 30, with wide variation between systems and between people.
This table is the practical heart of the chapter. All figures are averages for people who are not deliberately training, and the variation between individuals is enormous.
| System | Typical decline | When it becomes noticeable |
|---|---|---|
| Aerobic capacity (VO2max) | About 10 percent per decade after 30; roughly half that if training continues | Breathlessness on hills and stairs; the first thing most people notice |
| Muscle mass | 3 to 8 percent per decade after 30, accelerating after 60 | Difficulty rising from a chair, carrying shopping |
| Muscle power (force x speed) | Faster than strength | Inability to catch yourself when stumbling. The falls risk |
| Bone density | About 0.5 to 1 percent a year after peak; up to 2 to 3 percent a year for 5 to 10 years after menopause | A fracture, usually |
| Kidney filtration (eGFR) | About 0.8 to 1 percent a year after the mid-thirties | Nothing, until a drug dose or an illness reveals it |
| Lung function (FEV1) | About 25 to 30 mL a year after the mid-twenties | Breathlessness, decades later |
| Maximum heart rate | About 0.7 beats per minute per year, regardless of fitness | Lower exercise ceiling |
| Nerve conduction speed | About 10 to 15 percent slower by 70 | Slower reactions |
| Processing speed (cognition) | Steady decline from the twenties or thirties | Slower recall, more effort multitasking |
| Crystallised knowledge (vocabulary, expertise) | Stable or improving into the seventies | Nothing. This is the good news |
| Hearing (high frequencies) | Progressive from the twenties | Difficulty following speech in noise (Chapter 54) |
| Near vision (lens flexibility) | Progressive; crosses the threshold around 45 in nearly everyone | Reading glasses |
| Skin collagen | About 1 percent a year after 20, faster after menopause | Wrinkling, thinning, easier bruising |
| Immune function | Thymus involutes from adolescence; response to new antigens declines | Worse vaccine responses, more severe infections |
| Thirst sensation | Blunted | Dehydration risk, often unnoticed |
Three observations worth drawing out.
Not everything declines. Vocabulary, accumulated expertise, emotional regulation, and reported life satisfaction (which follows a U-shape, rising again after midlife in many populations) hold up or improve. The stereotype of uniform decline is wrong.
The systems that decline fastest are the ones most responsive to training. Aerobic capacity and muscle are at the top of the decline table and are also the two most modifiable items on it. That is not a coincidence; both are maintained on demand.
The threshold effect explains the suddenness. A person loses kidney function invisibly for forty years, then a routine infection plus an anti-inflammatory tips them into acute kidney injury. Nothing changed quickly except the margin.
Frailty: the state that actually matters
In short: A distinct, measurable, partly reversible condition of low reserve, which predicts outcomes better than age does.
Frailty is not a synonym for old. It is a state in which reserve across multiple systems is so depleted that a minor stressor, a urinary infection, a new drug, a small fall, produces a disproportionate decline. Two 85-year-olds can be entirely different: one runs a household, the other is destabilised by a chest infection.
A widely used definition requires three or more of five features:
- Unintentional weight loss (roughly 5 kg or more in a year)
- Self-reported exhaustion
- Weakness (measured by grip strength)
- Slow walking speed
- Low physical activity
Frailty predicts falls, hospital admission, disability, surgical complications, and death better than chronological age does, which is why surgeons and oncologists increasingly assess it before deciding on treatment. It is a better answer than age to the question "will this person tolerate this?"
And it is partly reversible. Resistance training plus adequate protein improves strength, walking speed, and frailty scores in trials, including in nursing home residents in their nineties (Chapter 7).
Healthspan and the compression of morbidity
In short: Life expectancy rose faster than healthy life expectancy, so the goal is shortening the gap rather than extending the total.
Global life expectancy is roughly 73 years. Healthy life expectancy, meaning years lived without significant disability, is roughly 10 years shorter, and that gap has been widening in several countries.
James Fries proposed in 1980 that the goal should be compression of morbidity: pushing the onset of disability closer to the end of life, so that the period of dependency is shorter rather than longer. This is the framework almost all serious ageing research now uses, and it is a better goal than maximum lifespan for two reasons: it is what people actually want, and it is far more achievable.
The evidence that compression is possible is reasonable. Cohort studies of people with few risk factors, particularly those who exercise regularly, find not only longer life but a shorter period of disability at the end, meaning they are healthy for longer and decline faster when they do. That is exactly the desired shape.
What accelerates ageing
In short: The same list as everything else in this book, which is the strongest evidence that these mechanisms are shared.
| Accelerator | Effect |
|---|---|
| Smoking | Accelerates lung, skin, vascular, and bone ageing. Roughly 10 years of life expectancy |
| Physical inactivity | The single largest contributor to reversible functional decline |
| Excess visceral fat | Chronic inflammation, insulin resistance |
| Chronic uncontrolled hypertension and diabetes | Accelerated vascular and kidney ageing, and cognitive decline |
| Excess alcohol | Brain volume, liver, cardiac, and cancer risk |
| Chronic poor sleep | Metabolic, cognitive, and immune effects (Chapter 10) |
| Chronic psychosocial stress and isolation | Measurable inflammatory and cardiovascular effects |
| Ultraviolet exposure | The overwhelming majority of visible skin ageing |
| Air pollution | Cardiovascular, respiratory, and increasingly cognitive effects |
| Repeated infections and chronic inflammation | Immune exhaustion |
What actually slows functional decline
In short: Four things have real evidence, and none of them is sold in a bottle.
| Intervention | Strength of evidence |
|---|---|
| Resistance training | Strong. The only intervention that reliably rebuilds muscle and bone at any age |
| Aerobic exercise | Strong. VO2max is among the best mortality predictors, and it is trainable into the eighties |
| Not smoking | Strong |
| Cardiovascular risk control (blood pressure, lipids, glucose) | Strong, and it protects the brain as well as the heart |
| Adequate protein, especially after 65 | Moderate to strong for muscle preservation |
| Sleep | Moderate, mechanistically compelling, few long trials |
| Social connection and cognitive engagement | Moderate observational; hard to randomise |
| Treating hearing and vision loss | Moderate to strong, and among the most neglected |
| Vaccination | Strong for preventing the infections that trigger functional decline in older adults |
One severely underrated item on that list is treating sensory loss. Hearing aids slowed cognitive decline in a randomised trial among higher-risk older adults, and cataract surgery is associated with lower dementia incidence in cohort studies. Both are common, both are fixable, and both are routinely left for years.
The science that is genuinely promising
In short: Real progress in the laboratory, no demonstrated human healthspan extension, and a commercial market far ahead of both.
| Approach | Status |
|---|---|
| Senolytics | Drugs that selectively kill senescent cells. Striking results in mice; early human trials in specific diseases; not demonstrated to extend healthy human lifespan |
| Rapamycin and mTOR inhibition | The most reproducible lifespan extension in mice of any drug. Human trials are small and short. Immunosuppression is a real concern |
| Metformin | Observational data hinted at benefit; the TAME trial was designed to test it directly and has struggled for funding. Recent evidence is more equivocal |
| Epigenetic clocks | Estimate "biological age" from DNA methylation and predict mortality better than chronological age. Whether they measure a cause or a consequence is unresolved, and commercial versions vary in quality |
| Partial reprogramming | Using Yamanaka factors to rejuvenate cells without turning them into stem cells. Remarkable animal results; a real cancer risk; very early |
| NAD+ precursors, resveratrol, and the supplement market | Extensively marketed, with human trials so far showing little or no clinically meaningful benefit |
The honest summary: the biology is real and progressing, the animal results are genuine, and nothing currently available has been shown to extend healthy human lifespan. Anyone selling you something on that basis is ahead of the evidence.
The Blue Zones, handled carefully
In short: A popular idea with genuine components and serious data problems, and worth knowing about both.
The "Blue Zones" (Okinawa, Sardinia, Ikaria, Nicoya, Loma Linda) were identified as regions with unusual concentrations of very long-lived people, and the lifestyle features described are plausible and overlap with everything else in this book: largely plant-based diets, daily physical activity built into life, strong social ties, and a sense of purpose.
The data problems are real and were not widely reported. Work by Saul Newman, awarded an Ig Nobel Prize in 2024, found that regions with the highest reported rates of extreme longevity tend to correlate with poor birth-record keeping, high poverty, and pension fraud, and that reported supercentenarian rates in several places fell sharply once birth registration improved. Okinawa's longevity advantage has also declined markedly in recent decades.
The reasonable position: the lifestyle recommendations derived from these populations are consistent with far better evidence from elsewhere, so following them is sensible. The specific claim that these regions hold a validated secret to extreme longevity does not survive scrutiny of the underlying records, and should not be the reason you do anything.
What ageing well actually looks like, by decade
In short: The interventions change, and the earlier ones are about building the peak while the later ones are about protecting the reserve.
| Decade | The priority |
|---|---|
| 20s to 30s | Build the peak. Peak bone mass, peak muscle mass, and peak aerobic capacity are all set now, and everything afterwards is decline from that ceiling. Do not start smoking. Establish habits rather than achievements |
| 40s | Catch the silent things. Blood pressure, lipids, glucose. Weight trajectory matters more than any single measurement. Begin colorectal screening. Keep resistance training or start it |
| 50s | Protect what declines fastest. Muscle and aerobic capacity need active defence now. Bone assessment if there are risk factors. Get hearing tested. Cancer screening as offered |
| 60s | Add power and balance work. Power declines before strength and is what prevents falls. Vaccination becomes more important. Continue everything above |
| 70s and beyond | Falls prevention is the single highest-value activity: strength, balance, medication review, vision, home hazards. Maintain protein intake. Deprescribe what no longer has a purpose. Protect social connection deliberately |
Sources and notes
Hallmarks of ageing: López-Otín et al., Cell, 2013, updated 2023. Organ decline rates are drawn from the chapters cited in each row and from standard geriatric physiology; individual variation is large and these are population averages. VO2max decline: Fleg et al., Circulation, 2005. Sarcopenia rates: standard geriatrics literature. Kidney function decline: Denic et al., NEJM, 2017. Maximum heart rate: Tanaka, Monahan, and Seals, JACC, 2001. Frailty phenotype: Fried et al., Journals of Gerontology, 2001. Compression of morbidity: Fries, NEJM, 1980, with subsequent cohort evidence including Chakravarty et al., Archives of Internal Medicine, 2008. Healthy life expectancy figures: WHO Global Health Observatory. Hearing aids and cognition: ACHIEVE trial, The Lancet, 2023. Rapamycin in mice: Harrison et al., Nature, 2009. Epigenetic clocks: Horvath, Genome Biology, 2013, and successors. Partial reprogramming: Ocampo et al., Cell, 2016. Blue Zones data quality critique: Newman, preprint and subsequent publications, awarded the 2024 Ig Nobel Prize in demography; Okinawan longevity trends from Japanese national statistics.
Open questions. Whether any intervention can slow human ageing itself, as opposed to preventing specific diseases, is unproven. Whether epigenetic clocks measure a causal process or a consequence is unresolved. How much of observed functional decline is intrinsic biology versus accumulated disuse and disease is genuinely debated, and masters athlete data suggest the second component is larger than long assumed.
That completes the foundations. From here the book turns to what goes wrong, starting with what a disease actually is. 👉