When talking about age I was answering a separate question, I did not mean potential was related to age.
I remember Charles definitely saying that 18 and 19 year old players train twice as fast as 21 year olds. This was said back in season 3 when we had the training speed increased (thus increasing salaries) to balance the excess money in the BB economy.
In my experience once a player hits 21 they start decreasing in pops in comparison to the 18 year olds. I have a cruddy 19 year old which I train in the same position as my best player who is 21. Training a single skill with both players getting 48mins my 21 year old takes at least 2 weeks more to go up a single level. My 19 year old is also the wrong height...