Oxidation is an inevitable consequence of our metabolism - we make energy by chemically breaking down compounds, which has the unfortunate side effect of generating a few still-reactive compounds since the chemistry is never perfectly precise.
These still-reactive compounds, in this context called free radicals, will chemically react with their surroundings or be quenched by antioxidants (or both, as it were). Sometimes, those surroundings include DNA or parts of the cell that interact with DNA. Damage to DNA is both permanent and cumulative, and with our technology cannot currently be repaired.
However, while this damage can certainly cause mutation and cell death, it isn't necessarily what causes the dysfunction of aging that we're familiar with. Because it can happen in so many different places in DNA, and happens differently in every last cell of your body, it means that some cells (and as experience tells us, most - for most of our lives, anyway) will survive. Unfortunately, survival doesn't guarantee sufficient fidelity for good function, which is why clearing senescent cells and cancers will be essential to reversing the damage of aging.
But even this clearance may not mean that our cells have properly functioning DNA and more types of aging damage resulting from that damage may become apparent over time. The ultimate defeat to aging will come in being able to completely rewrite and repair entire genomes and epigenomes, probably in a handful of new cells that are then grown up and reintroduced to your body.
(I'm only a lowly bachelor's in cell/genetics, so consider that last part wishful science fiction.)
Edited by Vardarac, 03 July 2015 - 02:09 AM.