This doesn't really make sense. Teen and young adult is a demographic/category used particularly in literature/TV shows/etc. Just because something ages, doesn't mean the demographic its officially registered under has to change as well. In this case with manga, it goes by magazines geared towards specific target audiences. Sailor Moon is always going to be a shoujo manga series geared for girls/young women. The magazine it was serialized under has it listed as such. Nakayoshi (this was the magazine, iirc) isn't gonna suddenly change its demographic just because Sailor Moon is 30 years old now and it would for whatever reason have to appeal to 30 year olds.
About the whole thing about voice actors, mangaka, etc.... I'm still not seeing the point. Voice actors, mangaka, etc. aren't the ones consuming this website's content, not are they the intended audience - consumers of their content are, as this was space was originally intended (an anime/dress-up/role-playing/hangout site). Now if somebody like that does join - and it has happened - it's not a big deal, of course. But again, they must bear in mind that this space wasn't made with them in mind. When these sorts of people did join it was usually for interviews/Q&As and marketing/promotion stunts with fans mostly in mind anyhow.
Besides that, there's nothing that indicates you can't consume such if you're not the target audience - instead, it means that if you're not part of the target audience, you should be mindful that the content probably isn't being made with you in mind. Anyone can join this web site and there have always been adults on this web site at every point in time. The ones that don't want to adhere to PG-13 standards, when they discover they can face sanctions for such, are free to leave for greener pastures.
But I suppose money also talks and if a given entity find that its consumers are mostly adults, they will feel more pressed to create content to appeal to them. Hence this site's problem. It's not an absolute requirement, though - that's a choice made by the company, typically driven by economic factors.
Otherwise, what are you trying to suggest in this thread? Do you want to see/discuss more R-18 content here? Because that's typically what makes the difference between a site geared for younger audiences and one geared for adults. Otherwise, what can't you do/discuss already that teens and young adults can't?