When did I become Ma'am?

As a teacher I'm usually addressed as Mrs. ..., but today (and it wasn't the first time!) someone sent an email to me and addressed me as Ma'am! I might add that this person was being respectful and I would venture a guess that he was not too far from my age! How did that happen? When did I become a Ma'am? You know in my mind's eye I see myself as a tall, young woman with long dark hair. Now today I've been jarred to a wide "aware" state by the word "Ma'am" and may need to change my self-perception. Ma'am what does that mean? Address of respect? It is only a word and a fine word that holds respect. Age, does it say anything about age? How could that little salutation of respect remind me of my REAL age? I guess I'll have to ponder this throughout the day to see how I feel as I dye my hair, take my calcium, and call my grandchildren. :]