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I know where this question and discussion are going, and agree with it, but I think it could go a little farther here by separating out the legal definition of an adult, which does include age, and what I think we're actually describing, and that's maturity.

Becoming an adult generally means being responsible and accountable for your actions. It also generally means that there are certain activities which are permissible when a certain age limit is reached that would be concerned illegal or at least frowned upon as a minor.

Which is why an age people can be married at have been set (unless other circumstances exist), as well as a drinking age. However, just because you've come of age, doesn't mean you're prepared, willing or able to except that responsibility and accountability, or responsibly participate in said activities.

So, does the act of turning 18 or 21 in and of themselves make someone mature, and knowledgeable enough to make good choices, as well as being responsible and accountable for their actions?

No.

Which is why being an adult and being mature, in my mind, are too different things. However, the law, and by extension, society, gravitates towards one size fits all kinds of solutions (while heaving every possible exception upon it). The more tidy you can keep it, at least on the surface of it, the better.

A 21 year old who still lives at home, and more or less dependent on his parents, is still considered a legal adult if he is mentally capable of making his own decisions and doesn't have some other impediment (outside of laziness or a general lack of commitment or direction).

And yet, there are teenagers, through emancipation, who obtain legal adult status before turning 18 because they were able to prove that they can function as an adult and, generally that being at home with parents would be worse than being on their own.

Age and adulthood come into play because of legalities and societies, but neither guarantee maturity, responsibility, or accountability. Nor does the designation of adult mean you're totally independent and able to live on your own, provide for your needs, make good decisions, or for that matter, take care of someone else.