I assumed she was a toddler - she seemed to have a very limited experience of interaction with other people, and obviously, she did not understand that winters come and go, which a 6 year old would certainly know. Also, her family, especially her parents, don't really seem to try communicating with her much; and there is no mention of her knowing anyone else. This could be because the story is told from her viewpoint and she is ignoring what people say to her, but it looks like while her parents and sister tell her things, they don't seem to expect responses or to ask her any questions. So she's either very young, or possibly she's autistic or otherwise has some communication/social disability.