Checkbox only visible when two other dropdownboxes is activitated

I have a site which handles signups for a competition. I want two of the checkboxes only to be clickable when the user have choosen some variables in two different dropdown boxes; box 1: 18 or above (option 3) box2: [login to view URL], [login to view URL] or [login to view URL] (3,4 or 5) This is the combination. it has to be option 3 from box 1 and option 3,4 or 5 from box 2. This is becuase powerbreaking and [login to view URL] is only for people above 18 of age, and with black belt ([login to view URL], [login to view URL] or [login to view URL])