So, did they do brain-scans of "teens" and "adults" in societies where the distinction doesn't exist? Seems like the critical piece missing from the argument. It's also possible that there was a smaller difference in the brain a few hundred years ago from factors like food availability. The current over-abundance of food is changing the the timing of puberty for example.
Is it food availability, do you have a citation - I hypothesised that this was due to the wash of oestrogen-like chemicals and hormone disruptors in domestic use in the more developed nations.
Edit: an example citation - http://www.telegraph.co.uk/earth/earthcomment/geoffrey-lean/...; other articles note the increase in the use of soya in countless processed foods and the high levels of oestrogenic chemicals it has.