I'm not sure it is age as much as personality and generation. A lot of my peers had grandparents as daycare, and quite a few of them did the same thing mine did with timing their retirement around their kids having kids. Which I think made sense, since few women in my grandmother's generation really had careers and most centered much of their lives around family caregiving. But I don't know a lot of grandparents in my mom's generation who were willing and able to do the same; most worked longer, and were less interested in giving up either the last years of their careers or the first years of their retirement to watch grandkids on a daily/regular basis. Part of that is probably the decline in pensions and the rise in divorces, which make early retirement a tougher thing to swing, but I think a lot of it was that our mothers did have degrees and careers and identities that are less oriented around motherhood so for them, staying home with a young child was a stage that they didn't necessarily want to revisit.
It'll be interesting to see how my generation ("elder" millennial) approaches grandparenting. Most of my kids and a lot of their peers say they don't want kids at all, though they're young enough - mid teens to mid 20s - that some may change their minds. My first and potentially only grandchild is 19mo now and I've yet to babysit her. I visit, I take her out with my bonus daughter, and I take BD out without the baby for some grown-up mental health time since she's a SAHM with few mom-friends, but I'm not really a baby person and prefer to leave the quality grandparenting time for when we can go to the playground or movies or shows or on adventures.