I think there should be a ratings tag for if people have a question on if a certain situation in their writing would tip them over an edge into another rating (Say, from PG to PG-13, or R to X?) Sorry for any confusion over ratings and stuff, if i could delete this, i would.
I would disagree that a ratings is required or tags for any specific rating value.
Given that the ratings mechanisms and the criteria that they apply are very different in different jurisdictions, I think it would be a very difficult tag to apply consistently.
The ratings you quote, R, X and PG-13, don't exist where I come from. Here in the UK we have rating systems for cinema films, video games and websites - all of which are different and are policed by different organisations. There are no ratings for books.
We don't get a lot of "R-rated and above" questions, so segregating them would probably mean they wouldn't get much attention -- people wouldn't be in the habit of going there. Plus, the boundaries can be ambiguous sometimes, especially on a worldwide site with diverse cultural norms. For example, in the US sex is considered "adult" but violence might only net an "adult supervision recommended" rating; in other parts of the world gun violence would be an instant "adult" but sex is more acceptable.
We do, however, have a mechanism to impede accidental viewing of material. If in your question you need to talk about "adult" content, however you define that, you can use the "spoiler" formatting, like this:
This is spoiler text. It is only visible on hover.
To do that, begin your paragraph with
>! (blockquote formatting + '!').