I would say even the 18T ones can be good, depending on if you have room in your Squad. For a few Seasons, I bought any of my youth that I had a spot for, and even though the 18T wasn't as fast a trainer as a 21 or 24T, he was a really good player (form averaged 8), his value was good, and he had two roles, so when I did eventually sell him a few seasons later I got a decent price.