I would say it works like this ...
10-14 years (baptized to make parents happy)
14-16 years (Hormones and questions kick in)
17-18 years (Married or disfellowshiped ... or both)
18-20 years (Divorced or going to college and exited completely)
So I would say, most between ages 18-20 ... although some come back for a breif time in their mid-twenties and leave again by 30.