I'm thinking like 1880s or '90s in a city like Chicago, Boston, or Philadelphia and working class. Public education? Any education? Working? Living with parents or out on their own? When would they be expected to start 'making a life for themselves'?
I should also clarify that this is based on an assumption of accuracy regarding the idea of the teenager as a post-war concept.
Didn't most cultures have gradual adulthood? Im pretty sure the Romans gave gradual statuses to children (infants, infantia majores, minores, majors).
Im just pointing ths out to illustrate that the concept that you suddenly become an adult is probably a major stretch.
Edit: Source
I'm going to give an account that some might label as belonging to the Critical school of sociology, and it probably is. As for the more 'ancient' history of the transition from youth to adulthood, I'm less than qualified to speak. Unfortunately, my answer will be based off of largely Canadian literature and statistics, but I can absolutely see parallels drawn to the United States as their economies developed in very much the same manner (at least at the time).
Around the turn of the century (1900), those between the ages of 18-24 were largely employed in the agricultural sector (almost half), and only a very small percentage attended secondary school. Another large proportion made close to living wages working in manufacturing or service industries. With these wages they could contribute significantly to their family, and this was often the case.
During the industrial era, those in their early teens had a significant amount of autonomy as compared to today. The reason for this is that they were absolutely crucial to maintaining production through employment in the industrial economy. Once their value to the economy was lost as production became heavily mechanized, the concept of 'youth' really started to arise. So in short, those in their 'teens' would have been seen as productive members of society, and therefore, much the same as adults in their capacity to provide labour and support for their family.
As for the concept of being either a child or an adult, that is largely true. The concept of the teenage years and adolescence is a very recent phenomenon. Due to the mechanization of production, youth are no longer of any value to the economy. As a result, individuals aged 16-24 (and even higher), are often economically dependent on their parents and lack any significant identity of their own. As the twentieth century wore on, this became more and more commonplace.
What we are experiencing today is an "institutionalized delay of adulthood" which is why the concept of a hard-working, economically independent 'teenager' may seem so foreign to us.
Sources:
Canadian Society: Meeting the Challenges of the Twenty-first Century Edited by Dan Glenday and Ann Duffy Oxford University Press
Dancing in the Dark: Youth, Popular Culture, and the Electronic Media by Quentin J. Schultze and Roy M. Anker