Yes, young trainees are an asset. They can help your team in multiple ways. One being the ability to sell for lots of money to buy players to contend. The next is the ability to have players play better than their salary ( given they are being trained). And another way they help is by giving you the opportunity to train them to fix weaknesses ( example-if your defense needs improved you can make it better).
If you do sell a young guy, make sure to have another to take his place to give you continuous flexibility.
I made the mistake of selling all my youth and bought older players to compete. However, during that time I didn't train either so my flexibility was destroyed. Then I was just hanging around in D.II with no way to get back to being a competitive team because my older guys couldn't sell for much so I didn't have any way to get better.