I wouldnt, no way. Simply because of his potential. Personally, my minimum potential to train someone is Perennial Allstar. That way i know he will still turn out useful no matter what league im in. Lower leagues he will star, later on he can still be a bench player.
You can pick up 18yr old players at the moment, I wouldnt be wasting weeks of training with someone that you wont be able to use in a few seasons time. But other questions come into play, like what are your long term plans, what level trainer do you have, do you have any other trainees at the moment.