The key phrase is "mentally competent." A person can be not competent for a certain period of time and then may be competent after the situation subsides, such as any medical crisis. This is particularly true for elderly people. Also there will be religious beliefs for the physicians who will be involved. Actually, physician assisted suicide will make physicians more vulnerable to make the decision, which will be very controversial unless clear laws are made to protect physicians from the family members, etc., for legal actions.