I think it depends on the country. For example, Mirai Nikki has ridiculous amounts of nudity and violence. It is raked R15+ in Japan and 18+ in South Korea. So I think EC anime would be ranked 12+ in Japan and 15+ in South Korea.
15+ I think too (although in France (sweet home) I rather feel that it would be 10+ XD (like all the unknown films are 10+ while sometimes it's hot (maybe not those containing sex but gore))