The 13 Hallmarks of Aging

Every cell you own takes on the order of tens of thousands of DNA lesions a day, and almost all of them are repaired before you wake up.

The damage is not the variable. Repair capacity is. The damage checkpoint halts a dividing cell until the break is mended, because a cell that divides through damage copies the damage forward. Antioxidant defense and the master antioxidant reduce the oxidative load that creates the lesions in the first place, and the longevity gene and the stress-resistance switch (FOXO) set how much of the cell's budget goes to maintenance rather than growth.

What accumulates is not one catastrophic error but the residue of a billion small ones — in nuclear DNA and in the separate genome the mitochondria carry. Genomic instability sits first in the list because most of the hallmarks beneath it inherit from it.

A cell keeps count of its own divisions, and the tally is written at the ends of its chromosomes.

Telomeres are repeated sequences that cap each chromosome and keep its end from being read as a broken strand. Most body cells cannot copy those ends completely, so a little is spent at every division. Protects DNA caps is the maintenance side of that ledger, and the longevity gene governs how much the cell invests in it.

When a telomere falls below a critical length the cell stops dividing and either retires permanently or dies. Telomere length is therefore two things at once: a record of divisions already spent, and a ceiling on the divisions that remain.

Two cells carrying identical DNA can behave nothing like one another, and what separates them is not the text but the annotation.

Chemical marks on DNA and on the histone proteins it coils around decide which genes are readable in which tissue. Those marks drift with age, so cells begin expressing the wrong program for where they are. The longevity gene family sits directly on this machinery, and the stress-resistance switch (FOXO) is one of the switches it acts through.

The drift is regular enough to be read as a clock — methylation patterns estimate biological age to within a few years — and epigenetic change is among the very few hallmarks that laboratory work has shown to be partly reversible.

A protein is a chain that has to fold into one precise shape, and a chain that folds wrongly does not simply fail. It interferes.

The cell runs a standing operation to keep its folded population intact: the heat shock response deploys chaperones that refold what has come undone, cellular cleanup and plaque clearance remove what cannot be saved, and antioxidant defense with the master antioxidant limits the oxidative damage that unfolds proteins to begin with.

With age that operation is outpaced. Misfolded proteins accumulate, clump, and obstruct the machinery around them — which is why proteostasis is studied as a shared upstream failure rather than a problem belonging to any one tissue.

The cell does not only build. It eats itself, deliberately, every day.

Autophagy is bulk recycling: worn organelles and protein aggregates are wrapped in membrane, delivered to the lysosome, dismantled, and the parts returned to stock. Cellular cleanup is that route. It is held in check by growth signaling and released when fuel is scarce, which is why growth restraint and energy sensing sit beside it here.

Capacity falls with age, so the debris the other hallmarks generate is cleared more slowly and accumulates. Macroautophagy was named a hallmark in its own right in the 2023 update, having previously been counted as one part of proteostasis.

Every cell has to answer one question continuously, and it is not a complicated one: is there enough?

A small set of linked pathways carries the answer. Energy sensing reports scarcity and shifts the cell toward maintenance; growth restraint is the brake on building when building is unaffordable; insulin sensitivity, the fat-burning hormone and the appetite & sugar hormone carry the same information at the scale of the whole body, and the longevity gene and growth & survival signaling set what the cell does with it.

With age the balance tips persistently toward growth and storage, in tissues that have no use for either. These are the most studied pathways in longevity biology, because interventions that shift them change lifespan in several model organisms.

Mitochondria were once free-living bacteria, and they never entirely gave up the arrangement. They still carry their own genome and still divide on their own schedule.

Mitochondrial energy is most of the usable power a cell has. Builds mitochondria is the renewal side — making new ones to replace the failing — and cellular fuel is the currency the process runs on. The mitochondrial pore is the switch that decides whether a damaged mitochondrion is retired, and fat-burning is one of the main substrates fed in.

With age output falls, proportionally more reactive oxygen species leak out, the separate mitochondrial genome accumulates its own damage, and the cycle that retires faulty units slows. The tissues that draw the most power — heart, brain, skeletal muscle — register the shortfall first.

Some cells stop dividing and then refuse to leave.

A senescent cell is arrested but alive, and it secretes — inflammatory signals, growth factors, protein-cutting enzymes — into the tissue around it. Clears worn-out cells is the removal route, damage checkpoint is what triggers the arrest in the first place, cell turnover is the replacement that should follow, and galectin-3 and ceramide load are signals that rise in the company these cells keep.

Senescence is protective when you are young: it stops a damaged cell from dividing. It turns costly when senescent cells accumulate faster than the immune system clears them, because a small number of them can disturb a great deal of surrounding tissue.

Most of your body is younger than you are. The lining of your gut is days old; your skeleton is replaced over about a decade.

That replacement runs from resident stem-cell pools, and the signals here are the ones that keep those pools working: tissue renewal signaling, developmental signaling and the vessel growth signal that supplies a regenerating tissue, with muscle, supports BDNF signaling and the bone breakdown signal marking where the balance between loss and replacement is read.

With age the pools shrink, divide less readily and differentiate less faithfully, so replacement slows across blood, muscle, gut lining and skin at once. Stem-cell exhaustion is classed as integrative — largely downstream of the damage above rather than an independent starting point.

No cell ages alone. The hallmark here is not damage in a cell but what the neighbours are told about it.

Cells coordinate through hormones, immune messengers, neural input and direct contact. Calms inflammation and immune support govern the tone of that traffic; blood flow (nitric oxide) is the signal by which vessels answer the tissue they feed; and estrogen, testosterone, lowers cortisol and the reproductive axis carry the endocrine half of the conversation, which shifts markedly with age.

The traffic grows noisier and more inflammatory, and signals that should stay local begin to travel — so damage in one tissue raises the burden carried by others. This is the hallmark that explains how a local failure becomes a systemic one.

Inflammation is supposed to end. The defining feature of this hallmark is that it does not.

Low-grade inflammation climbs steadily with age in the absence of any infection — often called inflammaging. Inflammatory alarm is the sensing arm, calms inflammation the resolving one, and eases pain is where the difference is felt. Tryptophan balance shifts under sustained inflammatory signaling, and galectin-3 rises with it.

Senescent cells, misplaced cellular debris and an immune system that regulates itself less precisely all feed it. Because it is sustained rather than acute it acts on tissue over years. Chronic inflammation was named a hallmark in its own right in the 2023 update.

You are outnumbered inside your own body, and the balance of that population shifts as you age.

The communities of the gut and the mouth lose diversity, change composition, and press on barriers that grow more permeable. Gut barrier is the one that separates those microbes from host tissue; gum pockets and attachment is the equivalent boundary in the mouth; and parasite burden and h. pylori are specific pressures on the same system.

Because these microbes take part in digestion, immune regulation and the production of metabolites the host actually uses, a disturbance there has consequences well past the gut. Dysbiosis joined the canonical list in 2023 — it could not have been listed earlier, because the tools to measure it did not exist.

Aging is usually described as something happening inside cells. A great deal of it happens in the space between them.

The scaffold of collagen, elastin and associated proteins progressively cross-links and loses elasticity. Reduces sugar damage addresses glycation, the reaction of sugars with long-lived proteins that drives much of that cross-linking; tissue remodeling and reduces scarring are the maintenance and repair side; and arterial stiffness, skin and galectin-3 are where stiffening is measured and signalled.

Cells read the mechanical properties of their surroundings and change their behaviour accordingly, so a stiffer matrix is not only a stiffer tissue — it instructs the cells inside it differently. This was proposed as an additional hallmark at the 2022 Copenhagen meeting and is not part of the canonical twelve.