It takes a long time to teach our kids because the system we have for teaching them is horribly inefficient and has been for thousands of years at this point.
Until around 1900, within the working class (which generally included lower middle class back then), education was reading, basic writing and arithmetic. A 12 year old - whether boy or girl - was expected to be a productive member of working class society and was often married (12 for girls and 14 or so for boys). Education beyond that was for the upper class (and upper middle class), especially university level education. Extending education through grade 12 (typically age 17 or 18) for (nominally) all young people happen since then.
When I was 18, I did not hear society complaining that 18 year olds were not ready to be adults. (Yeah, some parents had trouble accepting their kids were 18, but even they did not, in general, feel that 18 year olds were not ready to be adults)
Now, I hear a lot of complaints about 18 year olds not being ready to be adults (despite an increase in demand to try and sentence kids
So, what's different? Part of it is inefficiency in the education system. Part of it is that we need to learn more. And part of it is increasing societal demand for over protectiveness - whether by scaring parents or by telling parents "you can't" - or "must not" - let your kids do _____.
The biological reality is more complex. Teenagers (or more broadly, those from onset of puberty through full maturity) are neither adults nor children. And now, our society has shifted from forcing them to be adults to forcing them to be children. Yet, so many adults wonder why so many of today's 18 year olds are not ready to be adults.