Can suicide be morally justified in such circumstances

Americans are living longer than ever before, living with chronic pain and suffering that might not, at least for some, make that life worth living. The example of Admiral Nimitz and his wife, for example, is one of two people of sound mind choosing to end their lives.

Can suicide be morally justified in such circumstances, or do people have an obligation to continue living?  Defend your position.  If you believe people have an obligation to continue living, where does it come from? To whom is that obligation owed? If you believe that suffering justifies suicide in old age, what other contexts are there in which you think suicide is morally justifiable?