[...] why does our pool game fall off from what it was when we were younger? [...]
I hear this a lot--game not being what it used to be. And while there's probably frequently some truth to it, I think there's also mixed in some colored perception of what the game "used to be."
That being said, my one word answer is COMPLACENCY.
If you ask people who've been playing for 2-5 years about how they see their game and where they see it going, you're likely to hear goals and plans...want to move to the next level.. want to get to where I can play so-and-so even..want to get kicked out of the "B" tournaments. And there's ones they won't articulate--want so-and-so to respect my game...
It's much rarer, imo, for people who have been playing for decades to have such plans and aspirations. They tend to be comfortable.
It's hard to improve if you're playing 2-4 hours a week. But I think it's possible for intermediate-level players to improve playing as little as 6-10 hours a week. But you have to have an improvement-oriented mindset. You have to have goals and make the hours count.
I'm 48. I played as a teenager, didn't play for 25 years, and then picked it up again at age 41. I'm still improving, a fact that is really helpful when I play people that I see about once a year. People are always wary when they haven't seen young players for a while --maybe they stepped it up a notch. But nobody even considers that someone my age might have stepped it up.