Experience comes from playing time. Two seasons is about the time you need for a full level, if they've been indeed getting significant playing time.
In my early days I read something about players also getting experience from handling important gameaspects, as taking an important shot, or making an important play.So basically they get exp for playing minutes, and a little extra for important plays.I'm not sure if that text is still in place, nor that it (still) works that way.
what about my 18yrs player that have pitiful that start with>