> based on the brain scans of nearly 4,000 people aged under one to 90, mapped neural connections and how they evolve during our lives.

That is an absurdly small sample size to make such a conclusion.

It seems this age range could at least partly be culturally attributed. In modern industrialized life, many people don't have to "grow up" until a later age. At the risk of generalizing, people have more support from family, friends, and society at large.

Is the forming of those neurons based on some natural law, or is that people just haven't had to live the experiences that do so until their 30's nowadays?

As far as I know, forming neurons isn't something that "just happens". It happens due to catalysts in life. In pre-modern society, and indeed most likely in under-industrialized nations today, those catalysts, those experiences, would happen earlier. As others mentioned, there is a clear correlation with the typical age in which modern society gets married, settles down, and has kids.

I wonder what that era age would have been 200+ years ago.

>That is an absurdly small sample size to make such a conclusion.

Please show the statistical calculations in support of such assertion.

Not sure why this is downvoted. The authors themselves note that parenthood could be a catalyst for the change at 30-- in previous centuries, when parenthood happened much earlier, why could it not affect the brain's timeline? This study is simply descriptive of a particular dataset, a collection of snapshots at a particular time and place. Certainly the brain is elastic and responsive to external conditions.

What I noticed is that the 4000 samples are all from England and the U.S. Replicating this study with a greater geographical and socio-cultural diversity would be very useful in supporting or expanding these results.