I have been thinking this about grandchildren. My 2 DC are in their early 30s, and have no children yet, they say they may do one day, maybe... Could be 4-5 years yet... I am in my early 60s now, and am afraid that by the time they do decide to have kids I will be too old and infirm to enjoy them/do anything with them/help my DC look after them.
And before people say 'people in their 70s can be perfectly healthy and fit' the fact is that most people will not be super fit and super healthy at that point, and will not be able to deal with small babies/toddlers/small children. Even at just past 60 I feel that I would only just about be OK handling toddlers/small children, but not as well as I perhaps would have 5 years ago. In 5 years time when I'm in my mid to late 60s, I am going to be even less capable/able/useful.
I am also a little sad that I will not see them grow up. If they're not born until I am say, 67-70, I very likely won't see them get to their teens.
I do feel a little tinge of sadness when I see friends and peers (who are roughly my age) playing with and taking out their grandchildren, and taking them for beach days and the like. Some of these friends/peers/acquaintances are a decade younger than me, and have little grandchildren who they take to the park and the beach etc... As I said, I fear I will not experience this. (Until I am too old to enjoy it! Or already shuffled off my mortal coil.) I can't - and would never - expect my DC to pop out a couple of children when they're not ready though, just to let me have a couple of grandchildren to take out now and again.
There are 5 generations of girl and woman in a family near me. Great, great gran (Pat) who is 82, her daughter, great gran (Karen,) who is 63, 2 grans (Karen's 2 daughters) who are 43 and 44, (Alison and Kelly,) then the 2 daughters of Alison and Kelly (1 each) who are 24 and 20, (Emma and Poppy,) then 2 little girls aged 1 and 3 that the 24 year old (Emma) has. (Hannah and Lisa.) Poppy is still single and childfree.
They all live within 20 minutes walk of each other too - in my village. All grew up here... When I see all their holiday and BBQ pics and Christmas celebration pics on Facebook, I can't help but feel pangs of wistfulness and slight envy. I am happy for them, but would love to have a similar set up.
Comparison is the thief of joy and all that. Maybe things aren't as hunky dory and sweet as they make it out to be on Facebook! But how lovely to have such a big extended family, and grandchildren, (and great grandchildren etc,) all close by. Life was like this for me some 40+ years ago when I was much younger, and I would have been the grandchild and great grandchild. Most of my extended family - including my parents and aunts and uncles - are dead now though. Not many of us left!