You can do something like this currently with schema.org metadata:

    {
      "@context": "https://schema.org",
      "@type": "WebPage",
      "contentRating": "18+",
      "isFamilyFriendly": false,
      "audience": {
        "@type": "PeopleAudience",
        "suggestedMinAge": 18,
        "requiredMinAge": 18
      },
    }

- https://schema.org/WebPage

- https://schema.org/PeopleAudience

- https://schema.org/AdultOrientedEnumeration