Pinning down a single age rating for a 'Robin' adult anime is trickier than it sounds because ratings depend on where it’s released and what the content actually is. In general, anything labeled explicitly as adult — nudity, explicit sexual content, or graphic sexual themes — will be classified as 18+ in most countries. In Japan you'll often see 'R-18' or '18禁', in the UK content like that gets an '18' certificate, in Australia it's 'R18+', and in the US streaming or broadcast versions would be marked 'TV-MA' or a movie might be rated 'NC-17' depending on severity. Platforms also have their own tags: some streaming services use 'Mature' or 'Explicit' and sites like Pixiv or specialized distributors tag works as 'R-18'.
Another layer that makes this one complicated is the character context. If the 'Robin' in question is the Robin from 'Batman' or 'Teen Titans' (characters frequently portrayed as minors), any sexualized depiction can be illegal or prohibited even if drawn. Laws and platform rules treat depictions of minors very strictly, so many hosts will ban or remove such content regardless of artistic claims. Even fanworks that sexualize characters presented as under 18 often get age-gated, flagged, or taken down.
What I do when I’m unsure is check the distributor’s rating notice, look for explicit tags like 'R-18' or 'adult only', and make sure the platform allows that specific kind of content in my country. If I see anything suggesting a character is underage, I step away — it’s not just a rating question, it’s a legal and ethical one. Personally, I prefer clear labeling so I know whether to lock parental controls or skip it altogether.