Yes, you can add age verification.  First create a new page (that will be the age verification pop up)

• Add page (add any verbiage you wish to display on that page.  Then you will need to add the verify buttons...
  
• Next, change the frame type to use the "Button" frame type: 

•  Then choose the age verify ok button. (repeat steps to add the cancel button)

Once you create the new page, you can specify it in the site template and settings area

Here is an example of what the finished product can look like:

If you have any questions please feel free to contact support.