If you like data about cities (and who doesn't?) you won't want to miss this page which has a bunch of it. Above, see the age distribution for Washington, DC. See the enormous spike of college-age people. Marvel at the large number of excess women in their twenties. Then find yourself wishing you could find those numbers broken down by race or some measure of socioeconomic status so we could tell if that's because male professionals are more likely to move to the suburbs or because poor black men are getting arrested and sent to jail.