If you're talking about galleries, this could easily be accomplished using custom fields.

Add a new custom field to gallery entries "Content Rating". Use value chosen from a list. And populate accordingly with the list of ratings you would like to use for the content. Also mark required if you want it to be set on every gallery entry.

I was thinking about Galleries and I opted for a simple catchall "Content Warning" on videos which "may" offend someone on whatever basis (drug references, profanity, boobage) but then I thought this might be useful systemwide. For instance, perhaps a news article would discuss a topic that isn't appropriate for all viewers, or a download could be suitable for adults only. I know a lot of ISP's provide software to block problematic content and being able to work with those might be useful considering the inbuilt COPPA support.

Also I can never reference Custom Fields directly when added to Galleries, the given ID doesn't seem to work so I either opt for the default display of all fields in a table or just give up on the idea. I did it once, back in the day. Tried a few times since and it's just no.

Hmm so are you thinking more on the lines of content warnings? E.g. if someone wants to view something flagged with a content warning they get a warning and must click to proceed if they want to see it?

Yes, something like that would do the trick. All of the content on my website is public (not bothering with the headache of members) and for the sake of public decency it would be useful to be able to give people a warning before they proceed to view a topic/video/image/blog post, etc that they may have avoided given fair warning. Even for sites with Members I think this would prove useful.