Pop Up 'web access questions'

I design websites for wine companies, and in many cases they have to restrict website (or page) access to people who say they are over 18 years of age. So it usually looks like a pop up where you have to click yes or no to the question "are you over 18", before they can access the site, or specific alcohol-related pages.

I could see this would be very useful for 'code access' to certain content on the website too. ie. registered 'investor content'