Back in 1985 the original Gillick decision allowed the medical profession to accept consent given by mature minors as valid (in exceptional cases).
Paediatricians and other healthcare professionals often profess to work according to this. However this is often a fairly cursory (and unrecorded) assessment of degree of competence based partly on physical appearance. Often mature minors are asked to give consent while parents are present who are effectively giving implied consent. Whenever there is an option of recourse to parents the situation effectively becomes one of allowing consent ‘for show’. In such a case this amounts to no more than mere assent. If refusal to consent to life-saving treatment would not generally be respected, it can only be for treatments which could not be viewed as life-saving that any degree of real autonomy exists. Mature minors in addition cannot consent to blood or bone marrow donation or any cosmetic procedures.
If children are deemed pre-competent rather than incompetent they must surely pass through a finite point where competence is reached. If it is the competence rather than the gravity of the decision which matters (as in adults), then reaching this mythical point of competence is critical. When it has been accepted once, it should be accepted every time following that, however grave the situation. This may be used in a test case for refusal of consent, though so far there are very few such cases, where it could be very difficult to defend.