Good article in Slate, Evolution Is Finally Winning Out Over Creationism, by Rachel Gross. I have not checked the surveys myself, but Gross reports,
The people responsible for this shift are the young. According to a recent Pew Research Center report, 73 percent of American adults younger than 30 expressed some sort of belief in evolution, a jump from 61 percent in 2009, the first year in which the question was asked. The number who believed in purely secular evolution (that is, not directed by any divine power) jumped from 40 percent to a majority of 51 percent. In other words, if you ask a younger American how humans arose, you’re likely to get an answer that has nothing to do with God.
The overall proportion of Americans who believe in secular evolution has doubled since 1999, from 9 percent to 19 percent, according to a 2014 Gallup poll….[M]ost of that increase has been drawn from the pool of Americans who previously reported that they believed in evolution guided by God [theistic evolution], which simultaneously dropped from 40 percent to 31 percent.
Why? In part because evolution is “in the air” (thank the Internet!) and in part because evolution-deniers are older and dying off.
Acknowledgment. Thanks to Mike Antolin of the Colorado State University for the link.