I think it depends on the 13 yr old and the reasons the child is giving. In reading the story, it seems that the reasons for foregoing the treatment all come from the mother and not the child. They are also not based on Mgburg's alternative of suffer painfully for a short period and then die, or just die. They're based on an unverified belief that the standard treatment of cancer is inadequate and 'alternative medical therapy' is better. I don't think a 13 yr old is in a position to make that kind of judgment. It's the parent's judgment, not the child that is at the basis of this decision. So in this instance, if I were the judge, I'd trump the 13 yr old.
If the child was making Mgburg's MIL's decision, I don't think I would trump the child.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13