I have 2 allstars I have trained from 18 year old rookies, both capped at age 22 with roughly the following stats.
Jump Shot: prodigious Jump Range: strong
Outside Def.: wondrous Handling: tremendous
Driving: tremendous Passing: prolific
Inside Shot: respectable Inside Def.: inept
Rebounding: mediocre Shot Blocking: strong
Stamina: mediocre Free Throw: awful
Jump Shot: prodigious Jump Range: proficient
Outside Def.: wondrous Handling: tremendous
Driving: prodigious Passing: prolific
Inside Shot: inept Inside Def.: strong
Rebounding: pitiful Shot Blocking: inept
Stamina: average Free Throw: prominent
They would both be good enough to make your nations U21 team and even play in the top division as backups.