What age does the damage start? —Timber

I think you also have to consider the level of investment the person has made in the church. It's easy for kids to leave because the church hasn't yet cost them anything. Remember this from Carl Sagan:

One of the saddest lessons of history is this: If we've been bamboozled long enough, we tend to reject any evidence of the bamboozle. We're no longer interested in finding out the truth. The bamboozle has captured us. It is simply too painful to acknowledge—even to ourselves—that we've been so credulous.

As kids progress in the church, pay tithing, go on missions, make life decisions and even marry in accordance with the church's prescribed pattern for their lives, each step makes it that much harder for someone to see their way out.