I am 28 years old. I have been friends with most of my friends since I was in high school. I don't know how to address their parents any longer.
The friends I have made after high school I have no problem calling their parents by their first names. Because those people were introduced to me as "Mike" and "Jane" or whatever.
But people that I previously called "Mrs. BFF" or "Mr. Biffle" (because manners!) I cannot suddenly start calling them "Peg" or "Joe." I feel GUILTY addressing my best friend's mother by her first name, even though I literally do not know what else to call her because she's divorced now so calling her "Mrs. Marriedname" is incorrect, but calling her "Ms. Maidenname" seems like it's drawing attention to the fact that I don't call her by her first name. And I feel rude using her first name.
My mom just did a really good job teaching me manners as a kid, I guess.