Don't worry about it. Just fill out the application and ask the recruiter when you should call back. Make sure you don't leave even the smallest detail out of your background. If you have something from before you were 18 it is still on your record. If they find out something that you didn't tell them, they probably won't accept you because they think you were trying to hide it. Be completely forthcoming and you'll be ok. (unless you do have a record).