Many parents assume that the terrible twos start at two. They’re partly right, but this term is a bit of a misnomer. The truth is, these developmental periods start at slightly different times for different children. Parents will typically observe this characteristic shift in behavior in their children from around 18 months of age to 3 years of age. If you’re wondering if your child’s behavior is typical or cause for concern, write a few notes down about what you’re observing and speak to your pediatrician.