That's not my stance, in the slightest. But if you're going to make rigid rules that insist on individual's brains having the capability of a 'fully developed' one, then you can't ignore the fact that this development can be 'reversed' due to the consequences of age.
/19
In any case, if we're going to insist on adulthood being determined by this sort of biological absolutism, then in the biological sense, an 'adult' organism is one that is capable of reproducing. So, humans are adults as soon as puberty its. I.e. 11-12 years old.
/20