I don't think I've got this concept straight in my head because I can't imagine what it would be like to not want to do things and not be able, but I remember reading someone else's comments years ago about what it's like to be old, where you're sitting on a park bench in the sun and you're dreaming of doing great things and climbing mountains and when you open your eyes you have trouble just getting up off the bench.
Maybe you're over the hill when you start getting hurt all the time doing the things you like. That happened to my father very suddenly one summer. He liked cutting firewood and had done it for forever, all of a sudden he was making foolish mistakes. Cut a limb he should have known not to stand in front of, it whipped back and nearly took his eye out, knocked him flat. Got his truck stuck in the woods where he shouldn't even have had a problem and I had to chop it out with an ax. Wouldn't fix the handle on his sledgehammer and the head came off it when he swung it at a splitting wedge, went straight up about ten feet and nearly smashed in the back of his skull, hit him squarely.
So I guess you're over the hill when you make stupid mistakes and can't stop any more.