I've seen full makeup (Mascara, eye shadow & liner, blush) make a girl look older. I also had a friend who tried dressing oldr than her age (Slacks & tucked in, dressy blouses rather than jeans & forever 21 type tops) and some people did think she was older. Other, though, still thought she was young & needed to dress her age, so she didn't like it., The good news is, in a few years when girls are worrying about wrinkle cream, you'll already look younger...
i actually use to feel the same way and the fact that I have braces now makes me look like a little kiddo LOL I get carded all the time :/ I'm 19 but get told I look 16, seriously when you get older your going to be so thankful that you look younger then you really are. but if you want to try playing with makeup looks like a smoky eye look link or get a haircut that will make you look more grown like a bob.
Heyy, I have the same problem.. I know it sucks.. I'm 18 too, and everyone thinks my 16 year old sister is older than me. :/ not fun. but I don't think it really matters anymore, it kindof bothered me at one time, but now, I know who I am, and I know that I'm 18, and everyone else will know that I am comfortable with myself no mater how I look on the outside.. The trick is just trying to be confident about how you look, because there's nothing that you can really do to change that.. since age is iusually shown mostly in your facial structure. Believe what "yobootystank" said about how one day you'll like looking younger, like when you're 30, and still look 25 while everyone else looks their age.. we'll be getting a the 25 year old hotties hitting on us.. lol! there's always 2 ways to look at a situation, just try to look at it on the bright side.. =D
wear more makeup. use mostly natural colors but a darker eyeliner that kinda sculpts your eye and defining eye makeup can help. also using blush/bronzer to emphasize your cheekbones can help.