Don't know for sure, but from what I see, most city center residents live in flats. Iwould imagine that as people get older and start thinking about having a family they will want to move to a house with a garden and quieter streets etc!
Young people like living close to the centre with all shops pubs etc close at hand.
When people start getting older they prefere a quiet life and move outwards away from city centres. Plus as Oakleaf51 has said, they are the only ones who can afford to live there!
I live in a terraced house in a town centre and whilst I do like the location, I can't afford to live anywhere else so don't have much choice really.