Uh... Not necessarily. I just remember when I first started the game, before I could afford a bunch of younger trainees, one of my prime ones was a 22 year ole at... I think 6' 3".
But as far as things go, I've been training 2 of my current trainees for the last 3 seasons, and they're in their fourth season at 21 years now.
But as far as the percentages go, I can't be for sure.
These are just PURE GUESSES, don't take them to be fact...
age-percentage
18-100%
19-98.8%
20-95.5%
21-91%
22-85.5%
and gradually decreasing faster... at like 27 years of age, I can roughly say it's at or below 60%. I feel the decrease is like... exponential to an unkown exponent.