This is an extremely debated topic in the medical field. Arguments for it include bodily autonomy, dignity, and informed consent, while those that speak out against it point to the fact that in cases of patients who are under the legal care of a parent or guardian and are unable to speak for themselves regarding medical choices, assisted death is essentially murder (since they can't consent to it).
Overall, this is an opinion case that depends on one's personal ethics. In the interest of the human right of bodily autonomy, I say that in some cases, where the patient is entirely lucid, aware, and informed, they have the right to choose euthanasia.