Sometimes, even 3-year-olds just want to fit in with the group. Duke Today, May 27, 2021. What makes preschoolers eat their veggies? Raise their hand? Wait their turn? When it comes to getting kids to behave, by age three the voice of adult authority is no longer the only thing that matters. Picked up by WRAL, U.S. News & World Report and Futurity.