So agree with Jonesy here! I too, when in my 30's, teaching at U, had to use some of my clothing to seperate from my students because I looked so young. I especially hated if someone said I looked like 12: it is like an insult almost. 20 is flattering, 12 means "irresponsible", "child", "immature". It's not my fault, I thought, if I have no brows because they are so blond, if my face is round, if I'm petite!
In an effort to look older I painted my face like mad (too much), had my brows died regularly, and even, at one point, changed my hair colour to a much darker one. I wore suits, stayed away from mary-janes, flats, prints and feminine fringes. Well that was a mistake! First what does it accomplish? Not much. People still thought I looked young for my age. At 36, I was carded when buying a bottle of wine!
Today in my forties, I am starting to see the "old" Krishnidoux appearing, and although I await in curiosity, and I catch myself sometimes longing for those days when my only worry was to make myself look older (as opposed to scouting for wrinkle fillers, having to maintain my haircut more often, hesitating before buying black as a neutral, banning all lipstick colours except sheer pink, going to bed early too often...). On the bright side I don't need to put out such a conscious "mature" image when teaching. I wear very little makeup and concentrate on the cuteness, the appropriateness, the flattering of my outfits. I do this because I'm coquette but really, I could go teach in very bland clothes and still be able to project authority.
Anyway, sorry for the long post, all this to say: DON'T make yourself look frumpy, or wear outfits that don't match your personality, for the sake of looking more "mature". It is such a subjective perception anyway, it's not worth it. You are so pretty, and believe me, you do inspire respect from your image only. People who tell you you look like 12 are either jealous (yes, lots of those in the world. We may not want to believe it, but it doesn't make it go away nevertheless. People are selfish and if they feel insecure from your beauty, from your easy youth, they will say something to attack your self esteem just to make you insecure too. Not everyone is like this, but I'm just saying)... either they have no savoir-vivre and are so clumsy, that their distressing attempt of a compliment ends up being offensive. If you are sure that the situation is the latter, then it is not a bad idea to tell the person of their mishap. "I know that you want to compliment me and thank you for that, but 12 yrs old is simply too young of a reference here. It ends up being a bit insulting. I'm sure that's not your intention, though. Just be careful when you say things like that to people".