Yesterday's WSJ had an article (p. D1) about summer employment for teens. An excerpt:

This summer's teen employment rate will match a 57-year low set in 2004 and 2005, predicts Andrew Sum .... Just 36.5% of 16- to 19-year olds will be working, down from 37.1% in 2006 and 45% in 2000, he says, citing increased competition for part-time and temp jobs from older workers and immigrants.

I have a different explanation--leisure is a normal good. The rising affluence of families means their kids are less likely to seek summer employment.