Yes, I've been going through this with my ol' man Tipper. He's been to the vet several times already this year (blood and stool all good), and nothing is explicitly wrong with him. For which I'm grateful. But he's just getting skinnier and wobblier, and recently had a bout of puking and diarrhea that didn't help matters. I'm still working on getting that shaped up.
But yeah, sometimes I think they just fade a little as they age. Guess I should just count myself lucky he has lived to be an ol' man.