Young would make a lot of sense for the Celtics. With the NBA going small, he's no longer a tweeter forward and has become a high producing starting 4 in the league. Age wise he fits in with the Celtics core of IT, Crowder, Bradley, Olynyk, etc. Of course another huge reason Young would make sense for the Celtics at this point is because if the Nets lost him the Brooklyn pick the Celtics own this year could only get better. That assumes that Brooklyn with their new GM Sean Marks is starting a rebuild and would want a young player or players and/or picks for Young. If you consider the steep price it tends to cost to move up even one spot at the top of the draft, a slight overpay by Ainge and the Celtics to the Nets for Young could still make a lot of sense in the long run.
Since Young is the only thing the Nets have left from their trade with Boston that might complicate their asking price with Ainge. But the fact of the matter is the Pierce/KG trade was made by the prior Nets GM and Marks' responsibility is to improve the Nets future. Keep an eye out for either Sully or Zeller in any Thad Young deal.
You may wonder why Thad Young is getting more interest than Brook Lopez and it again goes back to the new NBA. Centers need to do two things in today's game: Defend the rim and stretch the floor. Lopez does neither. He's a great low post scorer in a time when no one wants that. Centers who can't stretch the floor like Lopez, Dwight Howard, Al Jefferson, Omer Asik, Roy Hibbert, etc have almost no trade value nowadays. Yes some can still put up big stats, but those stats aren't getting their teams wins.
Ainge would be wise to stay away from old school centers or big men in general. As you saw from the David Lee experiment a post playing big man only messes up the spacing in the Celtics offense. Thad Young on the other hand is a player who fits in with the new NBA.