I'd think that the "18+" type of categorisation would be _in addition_ to the Genre.
So for example you might have:
Horror -- (
horror theme, mild in terms of violence etc so suitable for all)
Horror 18+ -- (
horror theme,
that includes some violence or sex so suitable for 18+ only)
Mystery -- (
mystery theme, mild in terms of violence etc so suitable for all)
Mystery 18+ -- (
mystery theme,
that happens to involve some violence or sex so suitable for 18+ only)
And they I'd say that "Adult" (as in 'an Adult film') is actually a Genre, not an age classification. Because to my mind, "Adult" means that the key theme of the adventure is sexual and/or violent, rather than sex or violence being somewhat incidental to the plot. Obviously due to their nature, all Adult would be 18+:
Adult 18+ -- (
adult theme,
that involves some violence or sex)
Although to be honest I've not looked to see whether that's already how it's done - it just feels to me to be the way to do it!
Because I for one am usually happy to play an adventure that is 18+, but I would be very unlikely to choose to play one where the main theme is "Adult". Same as I watch 18+ films, but not the ones off the "Blue XXX" channel

psy