From the Pew Forum on Religion:

One-quarter of all adult Americans under age 30 (25%) are not affiliated with any particular religion, which is more than three times the number of unaffiliated adults who are age 70 and older (8%). Overall, younger Americans tend to be considerably less Protestant and far less religiously affiliated than older Americans.

Taking into account current census data (as of today), the number of under-30 Americans unaffiliated with any particular religion is 31,375,207. The harvest is abundant…