I have reviewed your project outline and can implement this script for you. The popup is integrated with a cookie script to achieve your desired result of only showing one.
Just have two questions for you. First, are you wanting the user to simply click yes or no to state they are of age, or have them enter in the birth date and have it computed? Also, is there any function (direct them to a new page, disallow access, etc) if they are not of age.
Thanks,
Ken