I think people think I'm above 20 yrs old.
Why do you think they think that?
My military uniform makes me look older
That is understandable. May be a good idea to correct them so they do not get the wrong idea about you.
I don't care. I want to get alcohol now.
Whether other people think this or not, it is still illegal for you to purchase alcohol under the age of 21. Be careful because it would be terrible for you to get into trouble for this reason.