Why does the risk of cancer increase as you get older?

Largely because cancer usually needs many genetic changes, and a long life gives errors in DNA time to pile up. Age is the most significant risk factor for cancer: it can strike at any age, but most patients with invasive cancer are over 65. Every time a cell copies its DNA, a few errors slip in by chance. Built-in error correction catches most of them, and a badly damaged cell can self-destruct in a controlled process called apoptosis. Only when those safeguards fail does a mutation survive and pass to the cell's descendants. Errors accumulated over a lifetime are one part of the age link, alongside the aging of the immune system and hormonal changes. As cancer researcher Robert A. Weinberg put it: 'If we lived long enough, sooner or later we all would get cancer.'

A single error usually isn't enough to cause cancer on its own. The real danger comes from errors accumulating in the same cell lineage over time. If a mutation happens to hit a cell's own error-correcting machinery, that cell and its descendants start accumulating further errors faster than normal. A later mutation might hit a gene that controls growth, making the cell divide more often than it should. Another might disable the gene responsible for triggering self-destruction, effectively making the damaged cell immortal. It's a chain reaction: each additional error lets the cell slip past one more of the controls that normally keep tissue growth in check.

Age isn't the only factor. Around a third of cancer deaths are linked to tobacco and alcohol use, obesity, low intake of fruits and vegetables, and inactivity, while infections from certain viruses, bacteria, and parasites cause roughly 16 to 18 percent of cancers worldwide. Genetics plays a smaller role than many people assume: most cancers are not inherited at all, and fewer than 0.3 percent of people carry a single genetic mutation with a large effect on cancer risk, together accounting for only a small fraction of all cases.