Rosie made a first "official" appearance on Harry's channel in a video entitled "FORFEIT FOOTBALL vs MY SISTER!" in 2016.
Her exact age at the time was unknown to viewers - unless they looked at her Twitter account which was unlikely. This should be disconcerting seeing as a vast majority of the comments from fans (on this particular video) did not appear to care about how old she was - instead describing explicitly what they would do to her should they end up alone with her.
This is not as disturbing once you realise that most of the comments come from prepubescent children.
Rosie is 17 years old, having just finished her GCSE exams of Year 11 (or whatever equivalent qualification they have in Guernsey - if any).