Well if there would be more age variants (18-22) there would be logically lesser of these 18y old drafts and as a ideal situation I would see that there would be like only one or two 18y old MVP or something like that. Well, if anybody want to create an ideal player, they have to do something for it.
I think that the point about that you can get 22y old player with some sufficient skills or have in draft better skills overall is logical, but is true that it makes a training even more worthless. And is good that if you have few skillups on a player, it in fact creates a difference.
If there have to be any differences, is the best way to have them downstairs (therefore basic is awful, best is respectable). So you can see more often that 5balls will get mostly players older than 18y and these above 20 would be like that almost everytime. Otherwise logically why they would apply for a place in the team so late, while they didnt get any improvement?
This is connected to theirs nature, maybe they should have different training speed, so if normal player is in his 22y at 70percent of his training speed, drafted player would be on 60percent, but would sustain like that until he will have 28 or like that. So make sense to train them, but maybe with a cheaper trainer and not very intensive.