At what age do you consider someone a adult in your OPINION?

I'd consider someone an adult when they are emotionally, psychologically, and cognitively mature enough to handle all adult responsibilities - such as supporting themselves and their dependents, contributing to their community, being able to deal with crises, and generally taking responsibility for themselves. A few people can do that by their late teens. A few never get to that point.

You're right. "Adult" is not a very specific term, and it is really only useful from a legal perspective. Beyond that, development continues throughout the life cycle. An adult of 18 is not the same as an adult of 30, who is not the same as an adult of 50 or 70. But they are all still adults. Maybe chronological "adulthood" should not be used as a criterion for granting all types of privileges, but it's hard to think of anything else that would work better.

Yes, I agree, 18 is still young, inexperienced. I think that a person becomes fully mature by 24-25.

My opinion is that some people NEVER become adults. An adult is a person who can take responsibility for themselves, their families and can contribute in a positive way to society.

Age has nothing to do with it.

Everyone matures at a different rate. I have seen 24 year olds be very responsible and some others are partying every night. In my opinion, age doesn't make you mature your actions do.

When the girl gets her first period she's an adult in my religion because her body can bear children and give birth.

A boy is an adult when he turns 14 years old in my religion.

In many other countries outside the West, children become adults when they hit puberty and are expected to get jobs, marry, have children and live like adults.