The only time I notice my age is it's tougher to play 3-4 rounds a day on road trips, and if they're particularly hilly or long courses, I feel it the next day, whereas I never felt it a few years ago.
I'll second this. I'm only 45, but I notice a huge difference in the number of rounds I can enjoy in one day. Back in my twenties, I played 120 holes the day I visited DeLaveaga. (4 x 27 plus some more holes until it got too damned dark to play anymore). I also remember playing 108 holes at George Wilson Park on the first nice day of 1998 (18 x 6!). I rarely play more than one round at a time anymore, might stretch that to 36 or 48 holes if I'm visiting a course with that many holes and I wouldn't want to leave without playing them all, of course.
The same goes for drinking beer! The fourth one definitely hurts now...Not the next day, but like the same friggin' night. Ugh.
On the upside, believe it or not, I'm a better player now. I used to muscle bug ugly forehands and tomahawks in my youth, still using that high school baseball arm, never taking the time to learn a proper clean RHBH form until 8 years ago or so. Gods, I wish I would have learned that 25 years ago.