I like the idea of doing this but I would expect making things complicated would likely cause problems so from my perspective the following fairly simple solution might work:

Everyone - suitable for all ages.

Shows up in search results.

No content warning regardless of logged on or visiting without logging in.

Example: some sort of flower garden simulator (grow flowers and arrange them in a garden).

Mature - more intense content.

Shows up in search results. 

No content warning if logged in.

Content warning if visiting without logging in.

Example: Some sort of ninja assassin game (sneak around and kill people but not with excessive brutality).

Adult - pretty much anything goes.

The default age rating.

Will not show up in search results unless logged in and a toggle in the profile setting its ticked.

Has a warning if attempting to access it without logging in (like it is already with most of these games).

Has a warning if attempting to access it while logged in and the profile toggle is disabled.

Example: A blatantly obvious porn game (no explanation needed I think).

Where tags are concerned I'm included to leave it to the developer to decide with users that are logged in able to suggest tags however the developer decides if the tag is appropriate or not - specific types of content could be noted here like drug use - to make it easier perhaps a tag cloud of some sort could be made to make tagging accurately easier.

This is a great discussion that needs to happen. Thank you to the Admin and Moderators for leading this effort to improve Itch.io and make it a safe space. I'd like to suggest that perhaps there is a deeper underlying issue underneath. Before coming up with a content/age rating system maybe it would be good to clarify what content is allowed (even within the adult section). I elaborated on this topic on a recent post that suggested a change in the Terms of Service. I encourage you to check it out and take action. Thanks!

Personally I'm more interested in highly specific trigger warnings than in vague age ratings.  For example, a player with extreme arachnophobia might have trouble with games that contain spiders, but be totally fine with sexual content.

Trigger warnings feel genuinely helpful to me.  Age ratings feel condescending.

I also stand against the implementation of a generic age rating system, and would be more happy with some kind of selective "in a category I may dislike" type of warnings system.

And of course there is a huge need for adding some way for users to personally blacklist or ignore games based on tags. At least I can say with confidence, that the "Adult" and "NSFW" section of the site is unpleasant to browse through without any options on content filtering.

My main point is that, people can be adults, with very different tastes in whatever. So a generic "you are 18+ so you are okay with anything" type of system wouldn't work well, and wouldn't improve itch website's current state in any way.

If itch decides to move to a generic, age-based rating system, then at least there should be a separate "For Adults Only" category besides the simple "18+" category, as it's possible to find things on itch with rather odd, uncommon, or extreme content, which surely doesn't pop-up in "simple" "18+" computer games. And here comes in the problem about defining what is allowed in the terms of service and the sensitive matter of artistic freedom and censorship, which I think that anyway can not be enforced without a sophisticated manually supervised curation system, which would at least be able to correctly recognize and tag all games on itch.

By the way, why is it not allowed for users to add tags to games? It would make it a lot more easier to search through the whole selection of games on itch, and more probable to find what people are looking for instead of totally random stuff, even without implementing a tag blacklist system.

so, we are required to add ESRB ratings or something like that???

I don't think this rating is meaningful on the Internet, because games can be downloaded and purchased at will. It can't verify what age people or children bought adult or pornographic content.

If it is a physical CD, the seller can verify the age of the other party (only need to submit the ID card).

In addition, I think that if there is too much adult content, this website will be blocked by many countries because it violates the minor protection law.

The best way is to lock the areas that do not accept porn so that they cannot access pornographic content. Only 16+(or less) content is open to these countries.

That's what STEAM does.

Don't put all your hopes on pornographic content. 

It is not the most profitable thing. 

PAYPAL also prohibits pornographic trading. It would freeze that income.

Maybe I am too old, but frankly, I do not give a  damn for any distinction for  the demographic of the    13-17yo.   You have to be 13yo to use itch per tos. So the only important age restriction is adult yes or no.

Since there is no curator system and no user tags that I would know of,   all games that would have to be   restricted by their content and not by their age appropriateness, would be dependent on the rating of the dev. And those would be    ...   questionable.

For adults that look for games to play with their children, there do be tags.  If it is tagged horror, for kids it ain't. And if the dev of a jump scare game    would not tag horror or similar, that dev surely would not have rated it correctly for any   maturity   preference.

It would be great if we could mute games site wide per tag, so if I don’t want to see any games tagged Adult or Erotic it will hide those results (loooots of NSFW stuff in the Top Sellers list* :P) You can add a button in the list labelled “Some search results are hidden per your muted tags”.

* I mean, good for them, but it’s not something I want to see every time I check out sales trends.

I do not like the idea of age ratings for games (or anything else, for that matter). There's no widespread agreement about "what's appropriate for a 16-year-old but not for a 14-year-old" or even "what's appropriate for 17-year-olds but not 12-year-olds." It's easy to discuss the broad outlines but impossible to find agreement on the edge cases - and labels that don't work on the edge cases are useless.

The most well-known age-based standards are used for movies. But games are not movies, and even if the same standards were attempted ("no more than one, non-sexual use of the F word in a PG-13 movie") (let's not think about what the rating "parental guidance" would mean in video games), it's unclear what that would mean in practice. Does "no more than one" mean "you can absolutely only encounter this once, IF you pick the right path," or can it mean "you can repeat the scene where this happens many times?"

"For adults only" runs into problems as well. Is that:

• Content the creator thinks is appropriate for adults (18+) but not minors (17 or less)?
• Content the creator thinks is illegal for minors? 
• ...in what country?
• Content that contains imagery that would not be shown on daytime TV in the US?
• Content that contains sexual/erotic themes?
• Content that contains extreme violence, gore, or crimes?
• Content that addresses mature, complex story themes?
• ...How do all these rules apply to TTRPGs or text-based games?

Having an official label for "Adult" content, as opposed to a creator-chosen tag, runs the risk of getting legal entanglements. Itch.io might be stuck being responsible for trying to keep minors away from the content - meaning they'd need some kind of age verification attached to the accounts. (Meaning: only honest minors would be blocked from the content. There is no way to do actual age verification online.) 

I'd like better filters and sorting. I'm very fond of AO3's method, but that's not likely applicable in other settings. AO3 has a short list of mandatory tags (including an option to not use any of them), and it relies on a large pool of "tag wrangler" volunteers to make help people find what they want and avoid what they don't. (At the simplest: Tag wrangling would mean making "roguelike" and "rogue-like" both show up in the same search. It cuts down on excess tags.)

I have yet to see a rating system that had objective standards and was applied fairly. (AO3 gets around this with a lot of hand-waving for the edge cases - and by not having any attempt at age verification, just a clickthrough that says "I promise I'm old enough to read this.") I have never seen an age-based system that was willing to answer questions in advance about how a work should be categorized - it's always "put your work up, and if someone complains about it, we may adjust the rating."

I would  much rather there were better tagging & sorting options than an imposed rating system. Even without tag wrangling - give me something like AO3's filters: Let me choose a set of tags to see, a set of tags to avoid, and sort by price or downloads or date updated.