It kills me when people act as though a team must elect to do one or the other. Having a couple of young players on your roster doesn't mean a team is rebuilding any more than having a team of older guys ensures playoff success.
Just because a team is in championship contention, doesn't mean you cannot add young talent to the roster. Of course, you MUST surround them with veterans, so that they can learn the ways of the NBA and learn from their mistakes. Yet, it is possible to develop some young guys on the fly, while a team is winning.
Ever heard of Rajon Rondo (Celtics), Glen Davis (Celtics), Andrew Bynum (Fakers), Courtney Lee (Magic), Aaron Brooks (Rockets), Mario Chalmers (Miami), Carl Landry (Rockets) or even Luis Scola (Rockets). All have 3 seasons or less experience in the NBA, and all are significant contributors to playoff-level or championship-contending teams. In the case of Rondo, you may remember his team won it all last year, with him as starting PG.
Therefore, I don't buy this crap that it's a team cannot afford to infuse young players during their "window of championship opportunity". Utilizing this approach, if nothing else, can only strengthen the roster and extend that window. On top of that, it can be done cheaply.