Actually, I see it like that, too, and think that people have been quite rude to differentnameforthis. (Not the childcare part, obviously, because it is a hassle for Mintyy to take the children to see her PIL, and she's not bothered about the three days.)
I think most of the replies seem to be saying that their parents can still do all these things into their 70s and 80s, so Mintyy's should be able to as well. And they can't because they don't want to engage with their grandchildren, so they don't deserve to have a relationship with them. There just is no empathy for how people feel as they get older... just that they ought to pull themselves together, step up to the mark, catch up on their sleep some other time, etc etc.
Someone mentioned celebs at that age, and how well they were doing. Well, I'm a bit younger than Madonna, but no-one would ever compare us. 
I see it as PIL are genuinely unable to cope, and the three days is the last straw. Perhaps it's because I've spent some time around less vibrant people of that age, and seen them struggle through. We all think we'll be different when we get there - just as my children think they will be different when they hit middle age. Just as pre-children, I swore I'd still be partying every night when I had given birth...
As I said before, if there's another issue, such as the PIL genuinely not interested, that's a different issue, and a different question.