They’ve still got their marbles, but their world has got much smaller.
I think this is the perfect way of describing it. I think, when you see the younger generations passing you by - not just doing things that you don't care about, but many (routine to them) things that you simply can't comprehend, many older folk decide that, instead of bothering to try to 'keep up', they will retreat to what they know, understand, like and feel safe with and step out of the rat race. I'm only middle-aged, but I can already see myself doing this to an extent.
Obviously, the age will vary for different people - and my own parents were never lucky enough to get old; but I think it's a very sobering thought when it properly hits you that the time you have left is limited. Things that, at 30, 40, 50 might have been categorised as "I must/will/might sometime" become things that you realise you will never have the time left to do - whether because you are very, very old/ill, don't have the physical ability to do or simply decide to prioritise the things that are more important to you.
If you figure that you might realistically only have time to read, say, 50 books at most in your life, you aren't going to waste time with ones that might be meh when there are others that you know you will love.
I actually remember my lovely FIL commenting along these lines, when he was 70, that he hopefully had something like 15 years of life left at most, so he wanted to make the most of it. He is now 81, and still with us; but his physical and mental abilities have seriously deteriorated. We all very much hope that he will get to 85, but there's every probability now that he may not. It's not necessarily just how many years of life you may have left, but how many good years. It saddens me to say that, whilst he is still with us and a much loved part of the family - we hope for a lot longer yet - his good years have now all gone
We sort of echo this rationale ourselves when it comes to holidays, which we always spend in the UK. We figure that there are so many lovely places in our own country that we want to go to again/for the first time, and only limited free time to do so, that it seems silly to go much further afield and miss out on all of those opportunities that we know and love on our doorstep.