<<My children are being taught the same thing. I had better hear a ma'am or sir come out of their mouth any time they answer a question. I always check with their teachers every school year to make sure they are saying it there too.>>
Exactly!
The way I was raised here in the south, "Ma'am" is a term of respect. It doesn't have anything to do with how OLD you are. Anyone who should be shown respect (teacher, a professional job situation, parents, etc.) will receive "sir" and "ma'am" as a form of greeting.
I taught my child to say Yes Ma'am and Yes Sir. He is expected to use it in appropriate situations. It's simply good manners.
ETA: I just have to say, if my son, who is 17, dared call a woman of any age by the greeting of "Hey girl", instead of "ma'am", he would be in big trouble on the home front!