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.