this question has been debated and researched for some time
no matter where you get the data the news is not good
found this on http://www.conversantlife.com/theology/how-many-youth-are-leaving-the-church
These are the most recent and most cited studies that I could find:
88%: The Southern Baptist Convention's Family Life Council study in 2002
70%: LifeWay Research study in 2007 (LifeWay also found only 35% eventually return)
66%: Assembly of God study (again, I can't find the actual study, only references here and there)
61%: "Barna study in 2006 -- "Most Twentysomethings Put Christianity on the Shelf..."