Board Thread:Character Discussion/@comment-27010228-20160131195431/@comment-7345267-20160131211003

It's never stated in the novel, either, but it's generally assumed that she's around 12 or 13, as her father makes a statement about moving her out of the nursery, saying she's too old to still be there.

Ages aren't given for her brothers, either, although Michael is basically a toddler and John seems to be 9 or 10 ish.