Here's a very interesting article about the adult conversion process.
In it the author, Mike Fleischmann, a pastor from Southern California, sets out to learn how many Christians become so in adulthood (rather than childhood), and what is the process they take.
He speaks of the 85/18 rule which is an axiom that 85% of Christians become so before age 18. He concludes that:
"I must admit that the "85/18 Rule" was partially confirmed in my research. In fact 84.5 percent of evangelicals do accept Christ before that age. However, the statistic only holds true if they were raised in a home where both parents were Christians with either a high or moderate level of spiritual activity. If, however, they were raised without that benefit, the percentage drops by two-thirds. The rest of the unchurched make their faith decisions throughout the course of adulthood and even into retirement."
He also has some interesting observations about the conversion process for adults.