Long time since I used the site but wanted to chime in anyways.
In general I'm a fan of simplified systems and not slang nomenclatures, so I what I would to is to tag all the games as "All Ages" by default, and add an opt-in "Adult" or "Mature" tag/system. If needed, add a "Teenage" tag too. This system should be pretty visible/a major requirement.
Reports should catch devs that forget, decide to not tag, or simply didn't update their games.
I strongly suggest not using any specific numbers (like "18+" or "18 an up") since, as mentioned, both laws and culture are different by country; so I think that a bit of ambiguity would really help. Similar case to "NSFW", some cultures might have no idea what's that, better to stick to some pre-2000 terminology :P
As an added suggestion, we could also have a filter for Mature games (both see it and don't see it). Maybe have an opt-in setting to enable seeing them? Maybe that will help with some countries' laws? Some kind of futureproofing?
For now I have no opinion on the checkboxes for content. I think it will be good to have but I'd consider it an extra, after the other systems have been deployed.