And then last, there are "Rookie Teams." The player must be 20 or younger to play on the team. They can play in normal league games, but it is easier for them to play on your "Rookie Team" because they can train, but there isn't any problem if you lose. You could have a tournament for the Rookie Team against the other Rookie Teams in your country, but there is no prize, or the prize is small.
You answered to yourself already. The point that you will sacrifice results of a Rookie team for the training means that there doesnt have to be any rookie team at all, because what makes you think that competition against same training only-oriented teams would be fun?
Perhaps if there would exist any limitation that you cannot sacrifice your results so deeply, it would make sense. Otherwise would be (for me) enough to have here (for example) training camp for rookies.