People do not age out of their secularism: Recently, Razib and I both used GSS data to show what other surveys have been showing: compared to older Americans, the young are less religious. But it is possible that this is due to an aging effect. One might tend to grow more religious as one gets older and finds a secular life unrewarding. I have seen research before (don't recall where) that found support for this.
I did not. Taking 18-22 year olds in 1990-1994, I followed them up until 2008 when they would have been in their mid-30s:
Percent who never go to church
Any changes just look like noise to me. It's possible that religiosity might develop in one's forties or older, but I doubt it. If the current cohort of twenty-somethings stay consistent like this older group has, their share staying completely away from the pews will be almost 30%.