So pretty much what i said
However i think PA can change based on what Raikan has said
http://www.fm-base.co.uk/forum/football-manager-2013-best-players/83183-complete-ca-pa-thread.html
Notes on PA:
The age in which a player reaches his full potential is up to different influencing factors. We need to take in consideration the standard of your facilities, your training schedules, the standard of your coaches, the amount of first team football the respective players gets and of course the actual difference between the CA and the PA. If he can improve a lot, then there is a high chance that he will.
Unlike CA, PA is generally a static figure that does not change once a game has been started. However if a player has a serious injury in the game their CA can decrease so much that they will never potentially be the player that they they could have been. PA is the maximum CA that a player can achieve and it is not possible for a player’s CA to ever surpass his PA. This system reflects well on real life, as a player who can be promising in real life may never reach his potential. It also helps to maintain a relative balance of player quality throughout the gameworld as time progresses.