How do I make people treat me like an adult?

I'm about to turn 21 and I'm a broad 6'7" although people still seem to treat me as a child. I'm very timid so I believe that's why although It's not like I'm weak. Nobody believes me when I say something and I feel like I'm being pampered or lectured like I was 5 the littlest things amazes people and it's insulting.
How do I make people treat me like an adult?
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