A simple plugin that verifies if a user is old enough to enter your site. You can easily customize the plugin with options and it uses sessionStorage to keep from popping up again. The modal box is responsive and is uses an external CSS for easy styling.
Click http://michaelsoriano.com/jquery-plugin-check-user-age/ for instructions and options
and
Click http://rawgit.com/fearlessflyer/age-check/master/index.html for demo