Oh hun I know EXACTLY what you mean!
As it says in my sig, I'm 25, but I constantly get mistakened for barely 18 (which, since having DS, I think is complete and utter rubbish)
When I was in my mothers group, I was definitely the youngest by far. I hadn't turned 25 at the time, I was still 24 and the closest person in age was the next youngest at 28