Adolescents are simply those people who haven't as yet chosen between childhood and adulthood.
For as long as anyone tries to hold on to the advantages of childhood - the freedom from responsibility, principally - while seeking to lay claim to the best parts of adulthood, such as independence, he is an adolescent. [...]
Eventually most people choose to be adults, or are forced into it. A very few retreat into childhood and never leave it again.
A large number remain adolescents for life.
(Gene Wolfe)