This study seems like finding a way to quantify the well known and then twisting it to make a good headline.

People don't grow up until they need to. Of course you're gonna see college educated rich westerners delay whatever mental markers you're looking at. And likewise people who "stay active" seem to stave off the mental decline of old age.

Exactly this. With a comfy life, you can mature later. The more hardships and adversity one must overcome the faster that maturation happens. Particularly so when the hardships put one on an abnormal path.

Losing a close relative or a job is normal adversity that everyone will go through but not everyone has. Going through those other things while having a different philosophy or life ethos than those around you, thus also causing you to prioritize and pursue different things in life adds a different layer of challenge. That causes you to have to figure stuff out on your own and thus contributes to maturing in a different manner and at a different rate.