Why does the risk of cancer increase with age?

Ageing is the single greatest risk factor for cancer. There is no one simple cause; rather, it results from an interplay of processes that accumulate over decades.

DNA is constantly being damaged, from the inside by metabolic activity and from the outside by environmental influences. When you are young, repair mechanisms work reasonably well. As you get older, those mechanisms become less efficient and damage builds up. Cells with too many irreparable errors stop dividing, but they do not always disappear. They remain in a kind of dormant state and secrete substances that can drive surrounding healthy cells toward malignant behaviour.

The immune system loses some of its sharpness with age. It detects abnormal cells less reliably and clears them less consistently. At the same time, a low-grade inflammation smoulders throughout the body, which can further fuel tumour growth. The same applies to epigenetic changes, changes in how genes are switched on or off without any alteration to the DNA sequence itself, and to shifts in the composition of the gut microbiome.

Interestingly, not all ageing processes promote cancer. The ends of chromosomes (telomeres) shorten as cells divide more often, and stem cells also become exhausted over time. This limits how many more times cells can divide, which actually lowers the likelihood of malignant transformation. This mechanism may explain why people over ninety sometimes show a slightly lower cancer incidence, although that evidence is limited and the underlying causes have not been fully clarified.

The relationship also works in the other direction: cancer itself, and its treatment, can accelerate the ageing process. This is most clearly documented in people who developed and survived cancer as children or young adults.