There's also an age-related decrease in immune responsiveness. I've been looking at this in relation to T-cell reactivity. There are basically two types; one falls off a cliff around age 65. If you're wondering why there are big-dose vaccinations for the over-65 crowd, that's why.

OK, so cancer. Cancers start small, and there's some evidence that the immune system can mop many of them up before they get to a detectable size (2 billion-ish cells), but as that immune system becomes less effective...

But before that, there's DNA. As I understand it, there need to be four alleles changed for uncontrolled growth (two pro growth turned on, two suppressors turned off). And DNA changes accumulate.

Trivia: many of the cancer producing viruses are retroviruses, meaning they picked up the flipped alleles from another human long ago.

That said, I have to wonder if there might be a link between sirtuins (a DNA repair coordination enzyme) and incidence of cancer. Sirtuins consume NAD+ as a substrate (meaning it isn’t recycled). There’s ongoing research on boosting NAD+ (typically by taking orally or injecting a precursor) but there’s no convincing evidence that that actually boosts intracellular NAD+. If you’re thinking of a study in which lab animals had their NAD+ depleted to see if the cancer rate went up (yeah, I went right there) complete depletion is, as the researchers say “not compatible with life”.

Just thinking aloud.