In the UK younger teenagers than this, if they are deemed Gillick competent, can theoretically refuse blood transfusion. I'm not sure though that any court has upheld the refusal of life saving treatment - so far. The reason usually given is that at that age one cannot understand the process of dying, or the distress and suffering there may be.
There was this case a few years ago ....
http://www.telegraph.co.uk/health/healthnews/7734480/Teenage-Jehovahs-Witness-refuses-blood-transfusion-and-dies.html
A really tough decision - I would like to know what his life expectancy is from the point of view of his relapsed Hodgkin's Lymphoma. It is pretty difficult to give effective and intensive chemotherapy without the back up of transfusions.
It goes against the grain to give treatment without consent, but it's certainly not easy, and perhaps even worse, to watch someone die from extreme anaemia..