What defines adulthood-- mind or body?

This question was inspired by another I just saw, where a girl asked if you are considered a child up until the point you hit puberty.

I did not get what would be considered a woman's figure until I was 19 years old. I did not get my period until then, either. By the definition of my body alone, I was a child until I was 19.

However, by the age of 19 I was in college, in a long term relationship, and was legally defined as an adult. I also had the maturity of an adult (or at least not the immaturity of a child).

I'm just wondering what defines an adult in your eyes. Is it the physical changes of puberty or mental development? Is a 30 year old with mental deficiencies an adult or a child? How about a wise, intelligent, and mature 13 year old (I know a 13 year old boy who speaks and acts as though he's 30)?
What defines adulthood-- mind or body?
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