DNA damage theory of aging and sex
I've answered a related question, which was wondering why older people get more cancer from accumulated DNA damage and yet their children aren't immediately susceptible to it, here:
https://medicalsciences.stackexchange.com/questions/19915/why-do-age-related-dna-transcription-errors-not-manifest-in-children-as-readily/
Quoting from my answer there:

...a parent does not pass on all their mutations in all their cells, they pass on just one, which need not have any deleterious mutations at all, even if the parent has cancer themselves in some other part of the body.


If that one cell is defective in some way, it is less likely to be involved in creation of a successful pregnancy, and may lead to miscarriage. Therefore, the probabilistic process of reproduction tends to result in offspring from selection of healthier gametes.

There are in fact mutations that accumulate in gametes, but if any of those mutations are very serious, they are unlikely to support development of an entire new organism, and they are also relatively few. The burden of age on the genome is spread over all the cells in a body, and worst in types of cells that divide the most, when errors are most likely to occur.