For a ratings example, see ratings for the Doom series from PEGI and ESRB. (Note that the Doom that was rated is different from the Doom activity, although the game is similar. Also, Doom 3 is much more violent and explicit than the original Doom. 12+violence might be a good rating for the Doom activity.)
Some have suggested the MPAA's ratings for films, but that system is not usually applied to software. ESRB and PEGI were designed for software ratings.