Actually, I started training 18 year olds the season after I promoted to II.1. They did start out as a liability, but my team was still good enough to avoid demotion, and those 18yos are now on their U21s and will soon become my best players.
Also, BC Törööö, 4 time BBB champion, has been training a trio of SFs since the age of 18. They're probably not a long way off from becoming some of the best SFs in the world.
Some teams do buy older trainees to remain competitive, but a fair number of top division teams train players from 18 as well, though I don't know of any that two position train them. It can be a good strategy in the long term, especially if you don't have the kind of money to buy older trainees that have been trained effectively.