I, like most of my friends, were idiots until our mid 20's. Learning of all kinds takes time.I know that we have sort of a universal "18 is adult" thing around the world but I'm only 36 and I'd consider Cam a "kid."
My own personal metric is post-undergrad age = adult, but developmental psych and biochemistry have a loose consensus that true, mental adulthood sets in around 25, when our hormones and secondary effects of puberty settle down and become normalized.