The reason I wouldn't recommend it in a higher division is that it is hard to train 6 rookies and still win games.
That's why you don't train 6 (or 5) rookies but maybe a (1) 22 year old, a (2) 21 year old, a (3) 20 year old, a (4) 19 year old, and a (5) rookie 18 year old. This way, you can play the rookie in scrimmage or cup games. Then the next season, the 22 year old can be replaced by a new rookie.