Sir/Mam

I've always been told anyone older than me, or any authority figure should be referred to as mam or sir. However, I was recently in a position of accidentally insulting someone by calling her mam. She was shopping at a local grocery store and I needed to get around her so I said "excuse me mam". She started to get angry saying "don't call me mam I'm not old". I had things to do so I just apologized and moved along. She was maybe 20 and I'm 16. Even if she was my age if still say mam, and would unless she was clearly a child. Has this happened to anyone else?