I was quite young (around 24/25) but had to get used to being greeted as ma'am...I was
a 2nd LT in the Army so it was "mandatory." So, I didn't take it personally. However, I just hated it when some smartass, young enlisted guys would say, "Good morning, sir." I just knew they were doing on purpose (especially if I had a skirt on instead of pants). Now, after years of smoking, my voice is very low...now I occassionally get "sir" when I'm on the phone with some customer service rep! Ma'am would sound just fine at this point, lol.
I think the first time I felt "old" was when my great-niece turned 30 (and she was upset at being 30!!!).