I humbly believe the age structure goes a little like this:
You spend your baby years, toddler years, preschool/kindergarden years being absolutely innocent and full of wonder.
You spend your teenage years, gathering ideas about the world around you and starting to figure it all out.
You spend your young adolescent years, getting a better grasp of the world around you and understanding. You experience more of what life has to offer you at these stages, better or worse.
You spend your next couple decades of your adolescent years, the 30s and 40s, presumably digging your heels on careers you've found worthwhile to pursue or you're just juggling things the best you can in ways you only know how.
By the time you reach the 50s and then on, thoughts on retirement settle in, thoughts about how you're going to land your life during the wind down period settle in too. You could still be continuing your goals and ambitions through this point, but this kind of thinking will be coming up more frequently. This is technically the point where your life is just all but mostly over.
Because by the time you enter 60s, you're wearing down, you get concerned as to how you're going to make it even longer should you be able to, who's going to take care of you, what things around your life are you going to settle with and so much more.
Then even further, you're just waiting for death to come take you because there is barely anything to look forward to. You think it's going to be as simple as just kicking back in a nursing home and having as much ice cream as you want watching tv, well, very few people get that opportunity. Other old people, who've already made it in their younger years, are just finding ways to extend their lives because they can afford nearly anything.
How old are you, got experience?
Sounds like late-stage midlife crisis.
Although I feel with OP.
I am in my very late 40s and often catch myself thinking exactly the same.