I decided to post a few guidelines about what it would take to make the U21 at the moment.
They are broad guidelines. If your player is on the edge, though, please do not hesitate to send his skills.
For 21 yr olds, at C I would think that the cut-off is about $40,000 in salary. There are already 3 Cs on the team at 50,000+. At PG and SG, a salary of $20,000 is the minimum. Same goes for PF ($20,000) although I would want good secondary skills if his salary is close to $20,000 (ie: respectable-strong JS and some other guard skills that are above awful). For a SF type, I would look at anything above $12,000 in salary, especially if his outside and inside d was above strong.
For 20 yr olds, at C I would think that the cut-off is about $22,500 in salary. At PG and SG, 14,000 minimum. PF maybe $14,000 although again the same emphasis on good secondaries. For SF, basically I would look at anyone's player over $8,000 in salary if he is committed to training a SF.
Still trying to figure out the 19 yr old pool. For the moment, if you have a 19 yr old player with $6,500+ salary and star or better potential, just send me his skills. 
If you have an 18 yr old prospect, I would say he would need to have at least star potential for me to track him at the moment. That doesn't mean that he would not necessarily make the team in the future, it is just that I have to draw the line somewhere (please prove me wrong and contact the U21 manager when your player hits 20 and say "see - thenewchuckd doesn't have a clue!"). I'm also only tracking guys who had $4,000+ in salary at the start of the season. Although, for a SF trainer I would go with $3,500+.
how bout this guy?
neil hutchinson(C)
Age: 19 
Height: 7'1" / 216 cm 
Potential: perennial allstar 
Game Shape: mediocre  
 Jump Shot: pitiful  Jump Range: atrocious  
Outside Def.: average  Handling: pitiful  
Driving: atrocious  Passing: atrocious  
Inside Shot: average  Inside Def.: strong ↑  
Rebounding: average ↑  Shot Blocking: mediocre  
Stamina: awful  Free Throw: inept      
Experience: pitiful