Humans commonly have grandchildren in their 40s (less so recently of course). We get fragile in our 70s. I'm wondering about living decades in that fragile state, where cancer happens to be a lot more common.
That mostly doesn’t count, since humans still regularly have kids of their own in their forties (so the direct link to natural selection would overshadow any lower-pressure grandmotherly effects), so you’d have to push the window out to the post-menopausal period, which coincides with the period in which various cancers begin to be an issue.