Adolescence was still a well-defined (psychological) concept prior to child labor laws. The name for the concept was "Youth", which was distinct from childhood and full adulthood. Again, while the legal rights of adolescents (or 'youth') has varied over centuries, there is clearly recognizable adolescent behavior going back to the ancient world. If we think about it a little, I'm sure people can dig up good Roman references to the risk-taking, sexual exploration, and rebellion of young people who were becoming adults (whether their parents and society liked it or not).
Off the top of my head, I recall one of Cicero's Philippics chastising Antony for his adolescent escapades with Curio. In the Greek comedies, there is also a terrific scene depicting young lovers being interrupted by three old hags; a scene that would be situationally identical to the horror of finding your grandmother's friends flirting with your teen boyfriend via MySpace.
Yes, this is obvious, but its not what I'm talking about. I'm talking about a sociological/psychological classification which arose in the late 19th/early 20th century to describe dependent, institutionalized young adults who are prevented from participating in adult life. Adult life had started in the teens up until this point. Of course there are similarities between young adults of antiquity and adolescents of today, that's common sense. The difference is that young adults of antiquity were engaged in business and family, whereas adolescents of today are prevented from doing so and exist as a physically mature (or maturing) person in the same dependent and submissive situation that they have been in from birth. Adolescents are old children, created by arbitrary social and legislative limitations. Some people never mature out of adolescence.