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
No, I don't think any rational person in modern society would consider an 11yo an adult. There are countless other factors to consider
But that's tacit acknowledgement of the fact that hard biological boundaries as a decider of human development are a bad idea
[Hint hint]
/21