I did a questionnaire about the realtionship between intentions and drinking alcohol...but my sample has 5 ppts under 18. What should I do?

I think it's fine to keep different age categories. The point of views differ with age. Also male and female understanding differs. So, I will suggest you to keep it, but just do not make it lengthy. Questionnaires should always be short, clear and to the point.
Best of luck!