My thoughts on this one are similar to the 'What gestational age would you want your premie saved?' thread - would have to be in the situation to really know. That does make it hard for planning purposes though.
The rule of thumb USED to be age + percentage = mortality. We do much much better than that now with survival rates but the question is not what degree of burns but what deficits would you be willing to live with? We still cannot save those with large area full thickness burns - mortality also depends on inhalation. Burns occuring inside of an enclosed space almost always are complicated by lung injury +/_ inhalation of toxins.