There's the legal definition, and then there's the personal definition. Sure, you're legally an adult at 18 because the government says so, but if you're 40 and still live with your parents and don't have a job, I'd say you're not an adult.
Personally I'd gauge adulthood by behavior, but that's not easily quantifiable in a large population. Age is, or should be, a decent approximation, but culture fucks that all up.
18 being the age of majority is what we call a legal fiction. It's a way of not dealing with the complexities of what it means being an adult, ability to consent, maturity, and so on and just pick a somewhat arbitrary age cutoff that greatly simplifies legal matters.
And even then we ignore it sometimes, like trying a minor as an adult.
but if you're 40 and still live with your parents and don't have a job, I'd say you're not an adult.
What if you are 40 and have a job and your retired/disabled parents live with you? Does that make your parents not adults?
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