I guess it depends when one starts working out, what they're doing and how hard they train. For someone that has never worked out it could be their teen years. Huckabee may just have recently hit his physical peak. For me it was 17-20 for overall fitness, even though I didn't lift weights but I was tremendously fit. I probably reached my muscular peak in my mid 20's when I weighed 265-270 but my VO2 Max was crap.
mid twenties, but depends on when they start training. if you start training like a psycho at like 15-16, then youd probably peak at 23-24. if you wait till 18-19, then later 20s
my best recovery rates etc were when i was about 22 personally
I had two peaks 19 and 26 - if i can get my ass in gear I would like to peak again soon, unfortunately the older you get the harder you must work at it.