dont worry! I have exactly the same problem. Everyone says I look 16 or 17 when I'm actually 21. But it's really not that difficult to get over the age thing when you meet someone! If you're at a bar or night club, obviously you have to be of drinking age to get in so everyone in that bar will be of age. When a guy approaches you, he'll won't be worried about age. Also, when a guy introduces himself to you ask for his age, the natural response is for him to asks for yours, then you can tell him :)
People typically go for other people that are around their age. If you look A LOT younger, guys might just assume you are jailbait and not go for you. But if a guy knew your real age, and you just happen to look a little younger, I don't think it would matter at all.