They’re still not talking what you’re talking about. They listed a set of specific activities and behaviors they believe 29 year olds engage in to say they’re not adults.
They eat children’s food, have no money saved, no proper furniture, no hardships, and they ask their parents for advice. (Having parents you respect the opinion of and asking for advice is evidently childish).
That’s an extremely patronizing view on 29 year olds.
You’re talking brain development studies. That has nothing to do with adulthood.






So do you think 30 year olds should be considered children, legally? Some intermediate thing where they get some rights but not all?
Adulthood, as a human concept as opposed to a strict biological classification, is a medley of biological, legal and social definitions. Do you exist in society independently, or under the explicit social umbrella of your guardian? Do we find you legally capable of bearing guilt? Are you physically mature?
Can you answer those questions with an fMRI? We can estimate age with one, but that just gets back to where we are now. We can measure brain connectivity, which is associated with the frontal cortex properties we associate with responsibility. The inflection point we see is around 15, and the growth rate after that is largely subsumed by the margin of error between individuals. We can also see that the brain doesn’t really stop developing those connections.
None of that answers the primary questions of what constitutes adulthood for humans.
Given that the comment thread started with assertions about how 29 year olds act and behave in society and what’s to be expected of them responsibility wise, it’s clearly a discussion about the social aspects of adulthood, not the biological measurement of brain maturity.