Why do people call females who are already grown up “girls” when they are already a young “woman”?

females that are already at age 20+, people still call a female who is 25, 30 a girl, when they are actually a woman. Girls are aged under 18. Doesn’t it make the female immature?
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I’m not talking about this website
Why do people call females who are already grown up “girls” when they are already a young “woman”?
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