If you look in the editor, the young players have a negative PA. This is because when a save file is started a random PA number is allocated within a range for every of those young players.
I don't remmeber the exact ranges but it goes something like this:
-9: 150-180
-8: 130-160
Etc
IIRC, a -10 is 170-200 and for every full point difference from that it's a loss of 20 points on the min and max. So yes, -9 is 150-180, -8 is 130-160
That changes at -2 or -1 (so you don't go actually below zero), but honestly if you see a guy with a -1 or a -2 just run the other way
As far as what the PA means, it's the absolute maximum cap on CA. What CA is, is a single summary of how good that player is based on their attributes. Players whose CA is below their PA can still improve their attributes. They can up some stats and simply because straight up better with no tradeoff.
However players who are at the CA cap (where CA=PA or very close) can't simply increase overall stats any more. At that point, it's a tradeoff. Training to have superior acceleration or jumping may cost them a point of stamina or a point in their dribbling skill.
That doesn't mean a player with a CA = PA can't become a better fit for your team. If your team is built to play a tika taka possession game, it may be important to sacrifice some aerial ability or raw speed for passing skills or attacking movement quality. If your team is built around rushing wide and lumping crossing into a big target, training him to have good aerial abilities is probably going to make him more valuable than making him a better dribbler.
Sure the players overall quality didn't increase, but he became more valuable to your team because he fits it better. FM has really tried to up this level of importance over the years. It's much more about finding players that fit a given gameplan than just collecting the highest PA and waiting a couple years, or managing a big club and just buying the highest CA.