The most rapidly growing religious category today is composed of those Americans who say they have no religious affiliation. While middle-aged and older Americans continue to embrace organized religion, rapidly increasing numbers of young people are rejecting it. As recently as 1990, all but 7% of Americans claimed a religious affiliation, a figure that had held constant for decades. Today, 17% of Americans say they have no religion, and these new "nones" are very heavily concentrated among Americans who have come of age since 1990. Between 25% and 30% of twentysomethings today say they have no religious affiliation — roughly four times higher than in any previous generation.
Putnam, Robert D., and David E. Campbell. "Walking Away from Church." Los Angeles Times, October 17, 2010.