The Dutch government published plans yesterday to legalize mercy killing under strict guidelines, which would allow children as young as 12 to demand and receive euthanasia. The plans are expected to gain parliamentary approval next year, which would make the Netherlands the first country in the world to legalize mercy killing. Under the new law, which formalizes practices already widely accepted and carried out in the Netherlands, euthanasia and physician-assisted suicide would be legal only if carried out under strict guidelines.