I'm about to turn 32 and people always think I am still a teenager. I also get the goggle eyes and mouths agape when I tell people how old I am (and at my job, my co-workers wouldn't believe me until I showed them my driving license with my date of birth on it). I don't really mind this, as I feel younger than I am and am very socially naive, so it's even a bit of relief that people don't expect the level of social ability and life experience from me that someone in their 30s would usually have.
For some reason there does seem to be something of a tendency toward larger, wide-set eyes and a "baby face" in autistic people (I've noticed people who've worked with a number of autistic people often say this). As genetic factors seem to be one aspect of autism it's possible there is some genetic link with these traits. However, I think some of this phenomenon is simply down to the natural expressions worn by autistic people - a wide-eyed dreamy, distant or bewildered look is common because of the way people with ASDs experience the world. This will give an air of youthfulness even to people who might otherwise look their age, and people who have a more age-neutral appearance will be assumed to be younger.