Why do people call me a kid when I'm 21?

Anonymous
This question is out of curiosity more than anything else. 18 is when you are technically not a kid anymore, legally. 21 year olds have every adult responsibility and privilege. In case someone says it is because I probably act like one, I'd say that I act like most people my age. But often when people hear my age, they may call me a kid.
Many older adults like to feel above or superior to younger ones
Because people that age are mostly kids
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Why do people call me a kid when I'm 21?
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