Why would surviving longer in old age be adaptive? The trade-off could be as simple as "you're mostly infertile, any children you do have will have higher rates

Why would surviving longer in old age be adaptive? The trade-off could be as simple as "you're mostly infertile, any children you do have will have higher rates of genetic disease, and being healthier means you're alive and active enough to compete with your kids and grandkids for food and mates".