I used to get this when I was younger, people thought I was older than I was though things seems to have changed as I've gotten older.
I've been ID'd for cigarettes numerous times over the past few years (usually on a no make up day) and when people try and guess my age I tend to get early to mid twenties when I'm 30 in November.
We used to do a quiz at my old workplace for newcomers as to how old everyone was in the team I worked in and often most of them were dramatically wrong, including putting 10 years on my boss (he never liked that game) and putting my colleague 10 years older than me when I'm actually 6 months older (he didn't like that game either).
I think it's nice to have a maturity about you, especially professionally whereas in your mind you can be as young as you want - as a friend said to me the other day, pick an age and stick to it :)
I commonly have people who assume I'm a secretary as I'm female and young looking and sounding, I know that "are you really old enought to be my lawyer look" well now :) I find it quite amusing and just smile and quietly confirm that I'm a solicitor and partner in the firm.
Even my secretary when she first came in thought I was another secretary as I "didn't look old enough" :)