I try not to go strictly by the ratings, they CAN be misleading. You can have a slightly better ID than thier IS but there could be one mismatch getting exploited.
I think the strategy of training younger players isnt just to produce top notch players, but to put cash in the coffers. In other words, I think the sooner you replace the older players you are training with younger guys the better. Just an old man's opinion though :)