I have 9 scout and above level players all under 20yo in my team. Half with special abilities. Done through power training. I try to bring 2-3 18yo's up to scout level each season, which means I can keep them between 5-6 stars easily for 4-5 seasons.
I have 13 scouts on my team, because I bought 5* close to 6* at the beginning of the season and trained them normally some are 30 other 31 and 32 ( I am level 2) and I will try to do the same this season