I propose that a special tag be created for the mature form of Cirno (チル姉 tag on pixiv), who is easily identifiable by her yellow scarf. I have noticed that other fan-creations, such as Keine's Sukusuku Hakutaku have been given tags and I think there are enough arts of this version of Cirno to warrant consideration.
http://www.pixiv.net/tags.php?tag=%E3%83%81%E3%83%AB%E5%A7%89
Updated by Keo