May I suggest you extension:141 to manage your adult content ? It will show a warning message in the top of the menu, it include a disclaimer, manage 3 kinds of user for guest (16-17 and +18) and manage group for registered users (nothing, 16-17 and +18).
This plugin do not (currently) show a message for each categories which are private, like "hey ! this is not empty ! porn content here !"

How you want do it when search, calendar, most visited, might show these pictures without awareness notice?


My solution:

Make these categories "Private"
Add a block by PWG Stuff for guest only to explain they must be identified with a generic user "over16" / "over18".
Create these users with no password as Generic users/

Authorise your adult categories to "over-aged" users. ;-)

Obviously any other member can be specially authorised (including you).
You can use user groups to reduce actions to do when you have a new category.