I don't think there is any consensus. Even across provincial jurisdictions, the age of consent varies greatly, from 14 to 18. I think capacity really comes down to the individual patient, with the individual question that's being asked of them.
In general, the less weighty the outcome of the decision, the more we allow the minor to play a role in that decision. For example, for a very young child, it may be asking which arm they would like their intravenous started in because the risk of harm is so low. However, a child not wanting potentially life-saving chemotherapy if they have a good prognosis or not wanting any further treatment if they have just suffered from a terrible car accident, for example, are the cases that are much more difficult to assess.