This is actually a pretty depressing question, since answering it forces me to catalog when my ever growing list of chronic injuries began. My peak strength/endurance and mental acuity were pretty clearly from my late 20s and early 30s - say 26-34.
After that injuries stated to accumulate and effect my ability to pursue different activities. Reduction in cognitive abilities has been subtler, but I started noticing a loss of my once photographic memory by about age 40, to the point where now, at 60, I frequently have to pause and rack my brain for a word or name.
So, long winded way of saying I'll take an eternal 30 - pretty please.