Here is a sample list of 18-23 years old "kids" I have tweeted at:
Kole Sherwood - Congratulating him for being the first central ohio native to sign with CBJ
Cam Atkinson - Scoring an amazing goal during a game
Ryan Johansen - OSU tribute during the ASG
Paige WWE - Being an awesome performer and hoping she keeps it up
Brandon Saad - Welcoming to CBJ
There is a common theme, none are college athletes and are already pros in their given sport.
Saying "you shouldn't tweet 18-23 year olds, it is creepy" as a blanket statement is, in my opinion, ridiculous. There is nothing wrong with tweeting out a congrats, a keep your chin up, or even a "that was a sick move/play/match".
What IS creepy is tweeting at them requesting favors of any type or attempting to get into their inner circle and it's fairly obvious that is the attempt.