For example, religious groups whose members are younger may be more likely to grow, not only because those members will live longer, but also because more of them are still of childbearing age (and thus have a greater chance of passing on their religion to their descendants).

With this in mind, some of the groups that have already been growing in recent years may be primed for continued growth. This includes people with no religious affiliation: The median age of adults who say their religion is “nothing in particular” is 38, while for atheists and agnostics it is 34.

Overall, these three groups together (often called religious “nones”) have a median age of 36 – fully a decade younger than the median age of U.S. adults overall (46), according to data from our 2014 Religious Landscape Study.