It's really very simple. Parents indoctrinate their kids into whatever religion they practice. Some kids drift off to another faith but there is little point in exchanging one fairy tale for another. I suspect that most of the ones who drift away really do not latch on to another faith and simply become the "None's" in the US.
http://www.pewforum.org/2012/10/09/nones-on-the-rise/
http://www.pewforum.org/2012/10/09/nones-on-the-rise/
Quote:“Nones” on the Rise
The number of Americans who do not identify with any religion continues to grow at a rapid pace. One-fifth of the U.S. public – and a third of adults under 30 – are religiously unaffiliated today, the highest percentages ever in Pew Research Center polling.
In the last five years alone, the unaffiliated have increased from just over 15% to just under 20% of all U.S. adults. Their ranks now include more than 13 million self-described atheists and agnostics (nearly 6% of the U.S. public), as well as nearly 33 million people who say they have no particular religious affiliation (14%).3
