I would consider that 18 is a bit too long to wait to be allowed to date. At the same time, if by dating you mean going out to a movie (just the two of you, dropped off by parents) then I think your parents are correct and you are too young. Of course, if by dating you mean simply considering the two of you "boyfriend and girlfriend", then I guess you can do that without any parental say so.
Finally, no matter what your parents say, I am a firm believer in following their rules as long as you live under their roof. If they really won't let you date until you are 18, then I would suggest you abide by that. But then I am old school and believe that parents should be obeyed in all things (unless of course by obeying it puts you in danger.)