If signed at 4* near the beginning of a season, a token youth can gain nearly two stars by the end of the season, simply because a 4* player gain progress faster than 5*.
if you buy a 18 y.o. 5* nordgen (with much less T) the first day, you can have him for 3 seasons as 7* player.
Also (because I do much training for many seasons) training progress it's a fix value and I never saw any relation between gaining and participation in the games.