Genomic instability
The DNA collects typos
Every one of your cells holds the same three-billion-letter instruction book, and has to copy it perfectly, again and again, while sunlight and its own chemistry attack the pages. The proofreaders are astonishingly good. Ageing is what it looks like when the typos start arriving faster than they can be caught.
Less than anyone selling you a pill will admit. What is beyond argument is what causes the damage in the first place: tobacco, sunburn, heavy drinking. After that the useful move is not repair but detection — the scans and blood tests that find the consequence while it is still small.
The science, in one sentence
Damage builds up in the DNA faster than the repair machinery can clear it, and some of it becomes permanent.
DNA repair
A maintenance crew working constantly on damage you never notice. Ageing is partly the crew getting slower than the damage.
Reduce the load rather than chase the repair: sun sensibly, do not smoke, keep alcohol modest, sleep. The single most useful thing here is not a supplement but screening — finding what went wrong early, which is the whole point of the Bridge of Prevention.
Oxidative stress
Rust, roughly. Some is normal and even necessary; too much damages what it touches.
The lesson of the last twenty years is that blanket antioxidant supplementation does not help and can blunt the benefits of exercise. Eat the colourful plants, train, sleep, avoid the obvious insults. Do not megadose vitamins to fix this.

