I was training a couple 22yo's this season and they're progress wasn't noticeably slower than the 19-21yo's.  If the player is a prime training canidate, I'd say go for it and train them.
My best player is 21 and I plan to train him at least another season or two.
                                
                                
                                
                                
                                
                                
                                    "Well, no ones gonna top that." -  http://tinyurl.com/noigttt