Looking at your question, yes it has relevance in MedicalSciences.SE but decisions on mental capacity for decision making is formed out of psychological studies.
Psychology is a branch of medical science which is why you can get cross-overs on subjects.
This question to me is purely about psychology rather than medical science in general and therefore more suited to Psychology.SE as originally thought by @Careygregory
As for practical and ethical perspectives, psychology studies things from the practically and ethically oriented perspectives. Ethics is at the forefront of all psychological studies. At least from the Western studies of Psychology. Being from Vietnam, maybe you are exposed to different forms of psychological study but nevertheless, Psychology.SE does not accept answers not based on scientific studies.