A person becomes an adult once they take responsibility for their own actions and the consequences thereof. Many people never reach that status, regardless of age.

It's not so tersely defined as that, I don't think. It's kinda like wax in a lava lamp of information. Some gloops, the others glop, until eventually you gloop and glop and feel confident. Rigid labels and markers on the spectrum don't really hold universally true, I'm finding.

It's not helpful to redefine terms separately from the article.