Here are some interesting data from an article by Mark Regnerus and Jeremy Uecker (2006--Review of Religious Research 47(3): 217-237).
Using data from over 11,000 adolescents (Add Health Study), they charted at what age the kids made significant religious transitions. Either they say that either religion is now very important where as the previous year it wasn't or they say they are now attending church much more often than they had been.
Two observations: As the authors note, there is a U-shaped distribution, such that age 13 and age 18 are the most mobile times of kid increasing their religious commitment and involvement.
Also, there are more transitions than I would have thought. Multiplying out the cumulative risk of getting more religious during the teens ends up with a pretty big #.
(Tomorrow I'll post data regarding religious decrease.)