Almost 20% of younger people think the earth is flat. From Scientific American:
Puzzled but undeterred, we used the information in the spreadsheet to calculate acceptance of the round Earth by age groups and found that only about 82.5 percent of millennials (as YouGov called 18–24-year-olds) agreed with “I have always believed the world is round.” That’s still dismayingly low, of course, but it’s not as dismayingly low as 66 percent. And those aged 25–34 turned out to fare a tad worse, with only about 81.8 percent agreeing.