I like the idea of doing this but I would expect making things complicated would likely cause problems so from my perspective the following fairly simple solution might work:
Everyone - suitable for all ages.
Shows up in search results.
No content warning regardless of logged on or visiting without logging in.
Example: some sort of flower garden simulator (grow flowers and arrange them in a garden).
Mature - more intense content.
Shows up in search results.
No content warning if logged in.
Content warning if visiting without logging in.
Example: Some sort of ninja assassin game (sneak around and kill people but not with excessive brutality).
Adult - pretty much anything goes.
The default age rating.
Will not show up in search results unless logged in and a toggle in the profile setting its ticked.
Has a warning if attempting to access it without logging in (like it is already with most of these games).
Has a warning if attempting to access it while logged in and the profile toggle is disabled.
Example: A blatantly obvious porn game (no explanation needed I think).
Where tags are concerned I'm included to leave it to the developer to decide with users that are logged in able to suggest tags however the developer decides if the tag is appropriate or not - specific types of content could be noted here like drug use - to make it easier perhaps a tag cloud of some sort could be made to make tagging accurately easier.