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.




This image seems to explain it.
Proton is the windows compatibility layer. Lepton is the android compatibility layer. FEX translates x86 (most desktop computers) applications to run on ARM (most mobile devices).