Where I am you need only be 14 to make your own health decisions. If there is a family disagreement (ex. religious beliefs) regarding medical care a legal guardian is assigned to the youth while a medical team and judge deliberate over the care of the child.

However, I think we are discussing a situation other than this. I am assuming you are referring to individuals who - for mental health reasons - have been deemed unable to make their own decisions. Age has no factor here.

You however have indicated you are a legal adult. Why is this relevant? Are you concerned you may lose the ability to govern your own affairs? Talk to you psychiatrist or therapist about this. They might be able to sort this out for you and put your fears at ease.