I wonder if that's true. There's bound to be some benefits or drawbacks to aggregate fitness when people age. Sure, the contribution is very indirect and so it'll happen yet slower. But imagine if people lived until they were 300 years old. Depending on how frail they are, that could be a drag on reproduction and resources.
But perhaps there’s social factor, like a better ability to protect offspring would pass traits down after DNA transfer.
If we consider grandparents, they could.
E.g. more fit/older grandparents -> more help and experience sharing for raising the kids given to the parents, more infants survive. This would chose for lineages where grantparents are helpful && live more.
What I meant though is that the main evolutionary pressure of us in in reproduction. Sure some past-reproduction-age traits play a role, but hardly as big.