Well, I think that is the natural life cycle of people going from young single college grads to married and family life. I love NYC, I like living there, I wouldn't want to raise a family there.

I'm not sure how much of NYC's growth is related to any larger trend, or if it is simply for the last 10 years crime has dropped like crazy because the police have done a great job tackling hard problems. Anytime you see crime dropping 50%+ it is going to change who wants to live there.