I don't really find it a problem to have trainees of a different age except I will either stop training the older ones and replacing them, sell them and replace them, or start training the opposite of what I've been training to round out my team. You should always try to balance the skills of your trainees, because their worth goes up since no one wants a player with prolific jumpshot, but no range or defense. I say just keep what you're doing and selling players you are done training/ keeping them and tryin to promote.