If I understand correctly, you are trying to create a contacts database with the option of designating a contact as either "adult" or "under 18".
Why not have a field in your database called f_adult (flag this contact as an adult). You can run finds based on that field and you can differentiate between adults and minors.
If you could upload a copy of your file that would be helpful in trying to understand your issue.
And if your contacts have a DateOfBirth field, the age can be determined automatically and dynamically.