I think the theory for training players until 22 is that you don't start training a player at that age. If you train a player from when he is 18 til he is 25 then that is fine, but if you pick up a rookie at 22 then you don't start training them then as it is too slow.
You say you have had 3 pops so far this season, but I think most managers would be frustrated with that. I know I would :P The only way I would be happy with that is if the player already had great skills and so each pop up was just adding a lot to the player. However, in contrary, I am training an 18 year old now, and I have had 9 pops so far. And I expect another 5 this season. So I consider that a major difference in training speed for the different ages, and probably why most managers stop after 22.