Telomere attrition
The lace ends fray
The ends of your chromosomes are capped, the way a shoelace is capped in plastic. Every time a cell divides the cap gets a little shorter. Run it down far enough and the cell stops dividing altogether — or becomes one of the troublemakers in number eight.
Very little you can buy, and the supplement shelf is a long way ahead of the evidence here. Nothing safely lengthens a human telomere. Smoking, carrying too much weight and years of unrelieved stress all shorten them faster, so the lever is your life rather than a capsule. And longer is not simply better: restarting the machine that lengthens telomeres is one of the tricks a cancer uses to make itself immortal.
The science, in one sentence
The protective caps on the ends of chromosomes shorten with every division; past a point the cell stops dividing or turns senescent.