Generally speaking most historians assume that the physical development of young people has changed very little over time - which means that the way young people's brains and decision-making capacities develop has always been more or less the same. There's one caveat to that, which is that there's pretty good evidence that the average age when children enter puberty has been declining steadily over the past 100 - 150 years. But apart from that, the process of mental/physical development appears to be quite consistent, both cross-culturally and across history.

So the simple answer to your question is that the "adolescent brain" hasn't really changed at all over the course of history - what has changed is the cultural and social role that society expects young people to fill between the onset of puberty and their early twenties.

Terms like "adolescent" and "teenager" are relatively modern inventions. The idea of the "teenager" only dates back to the 1940s, and the idea that adolescence is a distinct, special stage of life did not develop until the late nineteenth century, and did not become widely recognized until the early twentieth century, when a psychologist named G. Stanley Hall wrote a book called Adolescence, and famously defined adolescence as a period of "storm and strife" during which adolescents needed a lot of special supervision and guidance from adults. This is the so-called "invention" of adolescence that you were talking about.

Prior to that, the thing to remember is that the vast majority of people who were what we would call "teenagers" today entered the labor force at a young age and worked full time - so they would be expected to work, and to act, much like people who were much older than them. But they weren't necessarily considered "adults" either - adulthood was something that people achieved by getting married, having children, and becoming financially/materially independent, rather than something that you became when you reached a certain age.

So prior to the early twentieth century, people in this age group were most often referred to as "youths." History is full of many examples of people lamenting (and occasionally praising) the reckless, impulsive behavior of "youth" - so there are a lot of indications that adolescents sometimes showed a lot of the same behaviors that we associate with being a "teenager" today. But the really important thing to recognize here is that a lot of what we think of as common "teenage" behavior is only really common because of the cultural context that we live in. Young people today act like teenagers because our society gives them the time, space, and resources to do so - we keep them out of the labor force, we isolate them with their age-peers in schools, and we give them quite a lot of leeway to decide how they will spend their own time. As scholars like Grace Palladino have argued, our society actually depends on teenagers to serve an important role as consumers in our society; we actually encourage them to be trend-setters in terms of fashion/music/style, and there are whole industries that depend on teenagers as their primary market - and all of that affects how young people behave, how we respond to them, how they think and what we think about them. The very idea of "adolescence" is culturally-defined.

To illustrate this, contrast the life of a teenager attending school today with the life that most adolescent/teen-aged young people would have led prior to industrialization; they would have spent the vast majority of their time working, and with people of a variety of different ages (most likely family members) rather than at school with their age-peers. Someone living that life would often just be expected or even required to act more like an adult than we expect young people to today - in short, they had fewer opportunities to engage in the kinds of stereotypical "adolescent" behaviors that we assume are "natural" in adolescents today. [note, though, that this doesn't mean that what we think of as "adolescence" is any more "natural" than the life of a young, 16th century peasant - both are just different examples of how the process of human development can be incorporated into a cultural identity/role.]

The tl/dr, then, is that the psychological/physical development of young people going through puberty has always been more or less the same - but the cultural context that gives meaning to that experience has changed. Euro-American societies used to expect young people to work and act more like adults from a young age, so while they weren't really "mini adults," the experiences and behavior of "youth" was different. It's important not to confuse what we know about young people's mental/psychological development with the role that we expect them to play in our culture - as a consumer society that prizes and protects youth, that keeps young people out of the labor force and makes them spend most of their time with people their own age, we actually foster a lot of the behavior that seems like "natural" adolescent behavior to us. In a sense, they behave in certain ways because we as a society expect them to. History (and other cultures) provide many examples of how the process of human development can be compatible with completely different cultural roles and patterns of behavior for young people - and the real lesson there is that young people are certainly capable of acting a lot more like adults than our society expects them to.

Sources:

Kent Baxter, The modern age: turn-of-the-century American culture and the invention of adolescence

Sarah Chinn, Inventing modern adolescence: the children of immigrants in turn-of-the-century America

Grace Palladino, Teenagers: An American History