Legal age for guys in America?

I know my gram is just saying this to have me believe my bf can't do things, but I almost think she believes that in America (I live in oklahoma) girls are adults at 18 & boys are at 21. I know this isn't true or our miltary would be a hell of a lot smaller. She even said "Boys can't buy alcohol till their 21", my response "Neither can girls" with a what the hell look on my face. 1. Please just tell me she is wrong, just for the certainty & 2. Do you know of many other people that think this way?