Age Check layer for magento store

I need a age check layer for my magento store. If the user enters the page he must be asked if he is 18 or not. If he clicks on "over 18" he is forwarded to the shop. If he clicks on "under 18" he is forwarded to another page. This layer must only be shown once per session. The page runs with a ssl certificate. So it is necessary that no external scripts are requested. JQuery is installed. Full source code must be provided and necessary changes on all files must be documented to make sure that the function could be implemented again after an update.