1) people's tastes change, and as they age they may prefer other regions (ie upper east side).

2) lots of people move out of the city when they want to start a family. some think that suburbs are much nicer for raising kids (no need to worry about little kids going to school on the subway, etc)

3) as people become more affluent, they can move from less expensive areas (i.e. some neighborhoods in brooklyn) to more expensive areas. A lot of my friends started in greenpoint or park slope (in brooklyn) and moved into manhattan after a few years.

There's no regulation per se, but the city is so large and composed of so many small neighborhoods. Some of these are better suited for younger crowds and others cater to older crowds.