36
Mentally, mid 20's.
Physically, anywhere from my 30's - mid 50's depending on the day.
When I was in my early twenties, I laughed at all those guys going on about light days, their joints, and taking up things like kettlebells, yoga, and complicated stretching routines. Now I see it. Also sleep has gotten more important, a really great night of sleep is one of the greatest restoratives I've found.
Early 30's is when I found things getting hinky.