The U.S. Census Bureau says there are 83.1 million people between the ages of 18 to 34 in the nation. Studies show the average Millennial spends 18 hours per day using any time of digital media. And, 90 percent of young adults use social media, which is up from 12 percent in 2005, the Pew Research Center reports. Although research in brain development among Millennials is fairly new, some medical experts say the brains of people in this generation are physically developing differently because of their almost constant interaction with technology.