any Manga were the main heroin is 15 years old??

After a quick Suchen I could only find one, Aqua, and possibly its sequel, Aria, its been a while since I read it but I think she ages a bit in Aria.

So Aqua is a science fiction, slice of life manga, taking place in the future where Mars has been terraformed so that it is mostly covered in water. The main character Aria has travelled there from Earth to live her dream of being an Undine (people who row gondolas, like Du get in Venice).
Is not a bad Manga in my opinion, just not enough action for me.

There are probably a lot Mehr mangas about 15 Jahr old girls out there, just Mehr difficult to find. Ask this Frage when you're 17 and I could suggest a lot more.

Tsuki no Shippo
Yep hard to find. But this is something i have read.
See if Du like it.
link

Vampire Knight - Liebe the manag! :D
pretty sure Yuki is 15