The first critical period is around 3-4 years old and through Kindergarten. The prefrontal cortex matures, allowing children to begin to focus their attention, stop to think before they act, and switch perspectives and tasks. Children around this age begin to self-talk as a way to self-soothe and learn to share and cooperate with others. As children get older in their elementary school years, interactions with their peers and the ability to form more complex relationships becomes increasingly important.

The desire for independence and the formation of self-identity in relationship to peer bonds becomes more evidence in middle school and high school years. This is the age when a person is particularly impressionable to cultural influences and peer attitudes, as well as the earliest onset for substance use disorders and mental health illnesses that have the potential to persist across the lifespan.