The only things potential does are:
1. inflate prices on the transfer market! ;)
2. determine how much skill a player can have before he is capped.
The jozefK topic gives some good numbers on how the caps may be calculated. Essentially, the potential determines the maximum sum that can be reached for the player before the training is slowed because of the soft cap. But it's not directly linked to age; a 25 year old player with starter potential who has received little training could arguably might not have hit the cap, while a 23 year old all star may already have been capped. You would need to know the players' skills and plug them into the formula to determine whether the player is capped or not.