I am 21 people confuse me for being 14/15 HELP!!

I am a 21 year old female, 5'4 in height and of a medium frame. Here are scenarios in which this occurred: I went to a school library near my home some time ago. The librarian told me off for bunking off school. I told her I was at the University but you could still see the doubt on her countenance. A woman who had me measured for a bra asked me if I was 14/15 years of age! People at my Church still think I am at sixth form at a push. They even give me colouring paper meant for kids! I had to tell the usher that I was 21 years of age! People talk to me as if I am a child. A boy at my University called me cute and was talking childishly to me, as if we are not of the same age! I ordered some groceries off Tesco yesterday. The delivery guy did not want to hand over the groceries to me. He asked me what age I was, I told him my age but he still could not believe it. He asked for more proof, so I had to show him my passport! I have 14 year old boys and teenagers hitting on me. If this does not warrant help, I don't know what does. How can I be treated like an adult? I do not like wearing makeup, so I don't know what else can make me look mature. I do wear office wear and heels though but still people think I am a child! Even my friends treat me like a child. Please HELP! :(

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