Do you think 19 years old is considered a teenager or an adult?

I thought being an adult is when you turn 18 but some people say it's when you turn 20. if that's the case I thought I was an adult like 4 years ago so I now I feel like I wasted my teens if that's the case and feel old. I'm 23 now and I feel old as hell for some reason. What do you think.. Adult at 18 or 20? Cause I know some 19 year olds calling themselves teenagers still.
Do you think 19 years old is considered a teenager or an adult?
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