The principal criterion usually revolves around whether the minor truly understands the likely consequences of their decision.
In this area, JW's have been their own worst enemy, because the majority of rank and file members have been educated to believe that blood component therapy is bad medicine; it is never indicated under any circumstances; it does far more harm than good and that quality non-blood alternatives are always available for any situation.
When a minor is that badly misinformed it is not entirely surprising for a court to decide that they do not truly understand the likely consequences of their decision. Even with JW adults, it has been suggested that some form of non-interventional paternalism would serve the patient better.