Here's some information.
Canada: http://as01.ucis.dal.ca/dhli/cmp_ww/default.cfm#A4
British: http://www.gmc-uk.org/standards/whwd.htm
70. You must also take account of the relevant laws or legal precedents in this area17, which vary significantly between Scotland, England and Wales, and Northern Ireland. In particular you should bear in mind that:
- At age 16 a young person can be treated as an adult and can be presumed to have capacity to decide.
- Under age 16 children may have capacity to decide, depending on their ability to understand what is involved18.
- Where a child lacks capacity to decide, a person with parental responsibility for the child may authorise or refuse treatment where they consider that to be in the child's best interests.
71. The wishes of a child who has the capacity to decide whether to consent to or refuse a proposed treatment should normally be respected. However, the legal position in England and Wales means that, in some circumstances where a child has made a competent refusal of a treatment, a person with parental responsibility, or the courts, may nevertheless authorise the treatment where it is in the child's best interests19.
Some recent updates, court cases: http://www.bma.org.uk/ap.nsf/Content/WW2Update/$file/WWupdate.pdf
Here are a whole bunch of US examples, but you would have to pay for the full articles:
http://archpedi.ama-assn.org/cgi/search?qbe=archpedi;157/10/1035&journalcode=archpedi&minscore=50
Here's one that looked especially interesting: "Dying Young: Cues from the Courts"
"the courts employed either an autonomy-based (focusing on a minor patient's right to refuse unwanted medical care) or a parens patriae-based (focusing on an assessment by the court, in its role as parens patriae, of the minor's best interests) approach."
http://www.pitt.edu/~cep/81dyingyoung.html
http://aappolicy.aappublications.org/cgi/content/full/pediatrics;106/2/351
Australia:
http://www.anzspm.org.au/guidelines/limitations.html
Some more great links:
http://palliative.info/pages/Position.htm