To start with, I could never compare myself to an aging "pro". Let's just say at 54 years of age, I'm good on a good night and can suck with the worst of them on a bad night (in reality, I'm an APA SL5).
But the aging process has already affected my game. I have to shoot with "reading" glasses on to get a distinct, sharp image at the edge of the object ball I'm aiming at. When I'm not at the table I have to remove them so the table doesn't look blurry.
Then, almost 5 years ago I had rotator cuff surgery on my right shoulder (I'm right handed) and now have somewhat limited motion with that shoulder thus making "jacked-up" shots difficult.
Then, about this time last year I began feeling the effects of "frozen shoulder" (medical term: adhesive capsulation) in my left shoulder which I eventually had to have a surgical procedure on followed by about 30 sessions of physical therapy. Now I have somewhat limited movement in my left shoulder as well, keeping me from stretching out while bridging.
Two nights ago while in bed, I noticed my right shoulder was hurting. It is still stiff and a little painful today. I am starting to think I may be getting "frozen shoulder" on my right side as well.
On top of all this I have chronic stiffness in my lower back that effects how far I can bend at the waist. I load up on Ibuprofen about an hour before my matches start to loosen it up some.
So, with all that being said, I believe in my case I have reached the point of diminishing returns. If I would have taken pool more seriously (trying to learn more instead of just showing-off/banging) when I was younger, as I now do, there is no doubt in my mind that I would have been a much better player, then, as I ever will be now at my age.