When train a new position?

Hey guys.

I was reading that how much stars the player has, more the train slows, and I wanna train DL/DML for also raise "pass" skill for sake of my tatic strategy.

Then, I want to train 18 years player cause I can make a player with the key skills I want. But i have a doubt about the best time to train a new position, if i train a player with 5~6 stars, the progression of the new role training is also slowed?

If not, then is better train a new role or talent when the player get the 5 star for sake of a good cost/benefit?