It's hard to say as to why some do and some didn't. It is the mark of royalty, so perhaps they did receive the mark in regards to working as advisors. I'd imagine that Endymion wouldn't be able to get the mark because he's already royalty of another planet and is basically sailor earth. It could have been a conflict of power-literally.

Ooh, that's a good point. But technically, in the future, King Endymion becomes royalty of the Earth along with Serenity, he wasn't already the prince.  Who's to say?

But he didn't get a royal mark, did he? I still think it's because of his royal earth bloodline, but I think that's drifting into another possible theory.

This is not something supported by textual evidence, but this is my take on it. I think that Luna and Artemis received the mark (magically) as a sign of their allegiance to the Moon Kingdom. They are from Mau but they are working for Queen Serenity and her Silver Millennium, so I think the crescent moon is symbolic of their loyalty. I think that Diana inherits it from her parents because, by the time she is born, it is so much a part of them (at that point, they've pledged their lives to the White Moon and its descendants, basically) that it become hereditary. I think she was born with it.

^Either that, or she received it shortly before being given to chibiusa, since she was going to be, and is, chibiusa's guardian.
I have no idea how a mark would be passed down at birth, unless there was magic of the moon kingdom in their veins from being advisors for so long. How old were they when diana was born, anyways?

I just have a hard time imagining Neo Queen Serenity conferring a mark on a kitten... XD And I kind of explain it to myself as it was just such a part of them that it became more than surface physical and thus could be inherited. Full disclaimer: not real science. (:

Aurae wrote:

This is not something supported by textual evidence, but this is my take on it. I think that Luna and Artemis received the mark (magically) as a sign of their allegiance to the Moon Kingdom. They are from Mau but they are working for Queen Serenity and her Silver Millennium, so I think the crescent moon is symbolic of their loyalty. I think that Diana inherits it from her parents because, by the time she is born, it is so much a part of them (at that point, they've pledged their lives to the White Moon and its descendants, basically) that it become hereditary. I think she was born with it.

I totally agree! It might be possible that Diana received the mark after she was born, but I think it makes more sense if she was born with it (no matter how medically implausible that may be xD)

Well a couple things.   First of all the crescent mark appears on the forehead itself.  Tin Nyanko always has that part of her forehead covered with her hat/headdress so you can not say for certain whether or not she has a mark there.

Second of all was it not a thing that as a cat Luna was unable to speak when her crescent mark was covered up like by the bandage?

So with that said I have theory.  I actually do think that mark was a gift imparted by Serenity with the Silver Crystal not so much as a brand but to grant the power to understand/speak other languages.  Thus it's probably how Luna/Artemis were able to understand the language(s) used on the moon and on Silver Millennium period Earth (in Another Story which I know is not canon but it still fits the concept), as well as to understand/speak modern Japanese (and possibly English for her S movie journey to America).