This is really an ethical issue based on the notions of "informed consent" and "age of consent." Most of us would, I think, agree that very young children, under the age of five, for example, should not be able to make decisions about their own medical care because they are not capable of understanding the issues. We would also argue that intelligent sane adults, not under the influence of mind-altering drugs should be able to make such decisions.
The problem is with borderline cases. Should someone with Down's syndrome or early stage Alzheimer's or a twelve year-old be allowed to make significant life decisions for themselves? The problem is that every individual is different. Some 12-year olds may be very intelligent and mature and other 18-year olds immature and irresponsible. A law, though, must be universally applicable or it is not just.
Perhaps the solution to the dilemma is to appoint experts, psychologists and such, as is done in legal matters, to evaluate cases individually as a way of balancing the dual imperatives of protecting the innocent and naive and allowing individual freedom.