People over guess my age by 6 years every time cause I'm "mature"

It's usually when I first get to know them as friends and we're having a friendly conversation. Been getting this since I was 20. People always over guess by 6 years then when I tell them they go, it's because you're more mature.

It still happens even though I'm 29, which doesn't make sense cause 29 is a pretty mature age. Don't know what to make of it.

You believe them? Or is it cause I been smoking half a pack of cigs for 10 years?