I recommend you spend more time reading about jurisprudence. I’m not sure where you’re based, but in the US, for example, children below even age 13 can make autonomous decisions about their health when it’s determined that they have the capacity, even when the consequences may be life-ending. And, less relevantly, I personally believe this is the most moral option in such situations. If you have a 15% 1-year survival rate, all treatments either come with a small decrease in mortality, a large increase in risk for detrimental side effects, or both, and you can demonstrate to me that you understand the interplay of these factors, I don’t care if you’re 10 or 100, I believe you should have the right to bodily autonomy.