The stereotype is that boys with young faces really aren't as bright as children with more mature looking faces. And to compensate, baby-faced boys are always better behaved.
Wrong on both counts. According to the Journal of Personality and Social Psychology, boys with young-looking faces generally have higher academic scores than their older-looking friends, once the scores are adjusted for home and economic considerations and IQ scores.
But when baby-faced boys become delinquent, they can behave much worse than boys with an older appearance. Researchers say that's because they're trying harder, in order to overcome the stereotype.