age gate questions

Does anyone have a good strategy for creating an 'age gate' page. I am making a site for an M-rated video game that requires age screening. This requires a custom page with fields for a user to enter their birthdate. Depending on their age, they can then view content on the site or get blocked out from it.

I'm thinking of using variable_set() for this. For each page that is requested, a check is run to see if this has been set. If not, they are taken to the age gate page. If it is, they may see the page.

Is this a good way to go? Can anyone suggest any strategies for getting started with this?