Seven in 10 Protestants ages 18 to 30 — both evangelical and mainline — who went to church regularly in high school said they quit attending by age 23, according to the survey by LifeWay Research. And 34% of those said they had not returned, even sporadically, by age 30.
http://www.sliceoflaodicea.com/?p=626
http://www.usatoday.com/news/religion/2007-08-06-church-dropouts_N.htm