Young Car Expert are consumers between 16 and 25 years old. Now the largest generation in the U.S., millennials bought 4 million cars and trucks in the U.S. last year, second only to the baby boomers, according to J.D. Power's Power Information Network, which defines millennials as those between 21 and 38 in 2015. Millennials' share of the new car market jumped to 28 percent. In the country's biggest car market, California, millennials outpaced boomers for the first time.