Most Helpful Guy
I would need to see a picture to give an honest answer.
In general, don't be too worried about looking "sexy." The right guy will like you for how you are. You shouldn't change your look for any one person.
I know what you mean though. Some girls I know look like kids and even though they are only a little younger than me, look like middle schoolers. A lot of looking older has to do with how you act.
If you truly want to look more mature and have more sex appeal, ditch the comic books, mac and cheese, and any anime you might have. Start wearing make up, throw away the Disney t-shirts and tie-dye, go to the mall and get some fashion forward clothing. Start working out to lose the baby fat and tighten up.