Which reminds me that I recently joined Dreamwidth, which also has an interesting system. Namely, there are three levels:
- no age restriction;
- viewer discretion advised;
- age 18+;
and then there's a freeform text field "reason for age restriction". So you get the benefits of predefined categories combined with descriptive content warnings. It does raise the obvious question of where to draw the lines, but it's an approach to consider.