I just turned 39 less than 2 weeks ago. For me, I lose muscle faster if I don't work out. And it takes longer to gain back any strength I've lost. I hurt my rotator cuff back in June and took a break for a while from lifting. Its better now, so I'm getting ready for the usual beginning of the year cutting that usually brings 15 lbs drop, but that gets harder every year also. I also get hangovers that last up to two days if I go out and party. I don't do that often these days, though. Maybe twice in the last year. I've talked to friends my age, and they agree about the hangover part. Hangovers last longer once you get older.
And my gums are starting to recede, which isn't cool.
On the plus side, I still like my music loud, I still have my health, and just last month someone thought I was 24.