Risk taking during adolescent egocentrism may be primarily due to the underdevelopment of which part of the brain?

Pre-frontal Cortex (PFC)

The PFC is often one of the last regions of the cerebral cortex to fully develop. It is the area of the brain most associated with executive function, logic, reason, and some anticipation of consequences.

Research estimates that most females' PFC is nearly fully developed by ages 16-21; however, males lag in this development of their PFC by an average of ages 19-24.

Thus, before legal age (18), minors (males > females) are more apt to partake in risk-taking behaviors.