Yeah, in BB training terms, 22+ is too old to really start their training. You need to start at 18, maybe 19 years old.
Like Ohiobuck said, you want to start off with guys in the range of 3000+ salary. That starting salary, which is determined by their skills, is generally more important to look at than their potential. As long as potential is allstar or more, you'll have plenty of room to grow before their potential limits their training.
Note that potential doesn't affect current playing ability, nor does it affect training speed. So the "hall of famers" you have, who are old and have low salaries, I'm sad to say will never amount to good players.