The standard we have used for medical decision-making for youth and for mature minors—even though that's not really a legal term, as there isn't a real definition of that—is one that applies both for adults and for children.
Where there's some nuance is around the voluntary aspect of decision-making, because children and young people tend to make those with their families supporting them. Differentiating between a decision that is the autonomous decision of a young person and one they're making with their family is where we wouldn't want to see that decision being discounted because they're being supported by their family in making that decision.
That's the one tricky area, I think, but mostly it's the same standard of appreciating and understanding what is being proposed, the ability to make that voluntary choice, and understanding the gravity of the decision.
What we know from the case law around withdrawal of treatment and refusal of treatment is that when the gravity of the decision is such that the young person is refusing life-sustaining treatment, for example, a significant amount of scrutiny from the medical team is placed on that decision. That is just the standard practice, so I don't think we need to make it more difficult for the young person to make that decision legally.