People often wonder at what age children are most difficult. Using data from our recent survey of 24,000 parents, we get a clear answer: for moms, parenting feels very hard with a newborn in the house, and peaks when young children are entering school. For dads, 1-year-olds are especially difficult, but teenagers pose the greatest challenge. While there are some spikes in parenting difficulty at young ages, the hardest parts of parenting appear to be when children are making major transitions in their relationships with their parents—moms have a hard time as their children start to develop wider social worlds beyond the home, and dads have a hard time as children start to leave the protective space of the household.