I'm 34 this year and constantly being told I look like I'm in my early 20s. It seems to mostly be my face that looks young - all the women on my mother's side are the same. I have quite a "thick" smooth skin with very few lines or marks. And no kids/never smoked/never spent much time in the sun etc. Even when I didn't dye my hair and had a fair few greys, it made no difference. It happens whatever I'm wearing - from plain black dresses to skinny jeans and breton tops. I've tried different make up styles from natural to bold eye liner or red lips. I've worn glasses for over 10 years. Tried very short hair and very long hair. (Not all just for the purpose of looking older obviously!)
I try not to let it bother me but just recently had another experience of someone tell me in a work context there was no way I looked like I was in my 30s because I have such a baby face and it really knocked my confidence. 
A lot of people say "oh you'll love it when you're 50" and maybe I will but I sure don't love it now... I work in a job where having life experience is considered an asset and it really doesn't help when people are always saying "oh you probably weren't even alive yet when that happened..." and turns out they're talking about 1995 or something (I was born in 1983!).