You guys emhasize this 18yo thing. Is there really so much difference between a 18 and 19yo player?
(it is my first draft)
Well, there is no clear answer for that. (of course difference is one year, but that's not nice answer ;)
If you are selling and you want to get good profit, then yes, but also potential must be high to get good price.
If you training for your self, then maybe. It depends how multiskilled player you are going to make (training several skills high enough will take so many seasons...). Monoskilled monster player can be trained also starting from 19 years.
If you are training backups/reserve players, then age and even potential doesn't matter so much....
I have focused only for 18 years, because I will sell all youngsters - I don't have any free training positions.