An old quote sums up my feelings about Doc not playing young players. He's never had a problem playing young guys when his roster or that player's talent has warranted it.
As for Doc with rookies, I think Mike Miller, Rajon Rondo, Leon Powe, Glen Davis, Pat Burke, Tony Allen, Al Jefferson, and Delonte West would all be shocked to find out they didn't play as rookies.
and my favorite rebuttal to that is tell me which veterans (or players with more time in the league) were ahead of those players on the depth chart--especially for those players on the Celtics. Then, if you actually come up with anyone, subtract the ones that were out injured when those younger players got playing time.
you'll find you're left with no one ahead of those players that Doc could give minutes to -- with one exception, Sebastian Telfair ending up behind Rondo on the depth chart by the end of that season because Telfair played so poorly. And in that case, Telfair was still only a 2nd year player--not exactly a vet.
Ricky Davis, Gary Payton, Marcus Banks, Paul Pierce, and Jiri Welsch in the case of Delonte/Tony
Antoine Walker, Mark Blount, and Kendrick Perkins for Big Al (Walter was there though cooked at that point)
Ricky Davis, Paul Pierce, Raef, and Wally ahead of Gomes
Big Al, Gomes, Telfair, West, Tony, and Scal ahead of Rondo, Green, and Powe
KG, Powe, Pollard, Posey, Perkins, and PJ ahead of BBd.
McGrady, Outlaw, Garrity who were ahead of Miller.
Those rookies all ended up outplaying the guys ahead of them or at least earning rotation minutes.