Assertiveness is a sign of maturity because it allows for being able to disagree without being disagreeable. A mature teenager will be able to hold his position.
That is, it's something that teenagers will likely “mature out of” over time. The U.S. Supreme Court has tacitly acknowledged this emerging view.
It is a critical and pivotal point of time where a parent can steer a son away from childish thinking and move him toward more mature thinking.
Determining when a teenage brain becomes an adult brain is not an exact science but it's getting closer, according to an expert.