I’m beginning to partly agree with @Res but still not completely.
I’d suggest to divide readers on 18+ and less then 18.
Only 18+ could be available to read mature themes.
So my main suggestion here is:
Some writers like to write mature stories and some readers like to read them. Don’t limit that. Make that in your favor, Episode. I suggest you give us writers an option to mark our stories as >Mature< and >Mature+<
The readers who want to read stories marked as that, should be 18 + because this stories would contain stuff which is now not allowed by guidelines.
[ Additional I suggest you make featured Mature stories which would be payable, but not more than 2.99$ per month. ]