A few years back, blogger Michael Bell produced this clever visualisation of some US Pew Forum data, specifically changes in religious affiliation from childhood (top) to adulthood (bottom).
Notice that Atheism (red) is growing, although half of those brought up Atheist convert. Catholicism (green) is losing people both to Atheism and Evangelicalism (immigration keeps their total numbers up). Evangelical numbers (yellow) hold steady, but with a great deal of churn.
Bell also points out that most religious life decisions are made by age 23.