Something irritating happened when I was at a buffet

I'm 21M years old. But I have a baby face and I'm short. People often assumed I'm between 12-16. Recently there was a situation that happened when I was at a buffet with family. They have a sign posted, and say this numerous times over the intercome. They say that children 14 and under must be at the bar with a parent or guardian. I didn't think nothing of it. I was also wearing a lime green hoodie, and had the hood over my head. Sometimes in my opinion wearing the hoodie makes me look younger. But anyways this is what happened when I was getting my food. An employee walked up to me and said "hey, if you're going to be up here at the bar you're going to have to be with a parent, you can't be up here by yourself!" I told them "I'm not a child, I'm 21 years old!" They said "are you sure? Do you have ID I can see?" I went ahead and pulled it out and showed them. She kind of had a slightly shocked reaction but then she said and said "oh, okay!" And she went off doing whatever she was doing. When this happened to me it kind of pisses me off.