Why am I always being called ma'am? I'm 21 years old but everyone... cashiers, faculty at my college, random

Anonymous
people in stores that ask random questions (where is this, what time is it), etc calls me ma'am. I hear it from both men and women, but more often from men, ranging from my own age to fifties. And it's not the dress style, because I hear it no matter what I'm wearing whether it's a t-shirt and jeans, a fancy or even showy dress, professional looking outfit, short shorts, etc. Anyway not sure if this matters, but I am told quite often by friends, family, strangers, whoever that I look demure, calm, collected, sincere, and other similar-meaning terms. So what's up?
Why am I always being called ma'am? I'm 21 years old but everyone... cashiers, faculty at my college, random
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