This thread still lives. I will further elaborate on my earlier thoughts as new posters have not addressed these issues.
1) When
Numenor was set apart, they were awarded a kind of demi-Eldarin life. They were no longer plagued by disease and old age came much later. They also were able to tell when dotage approached and were "gifted" in being able to simply lay down their lives as did
Aragorn in the LotR.
2) As portions of the
Numenoreans became corrupted, they lost both life-span and health. It appears that this fate was not sufferred by
the Faithful. They continued to have longer lives and they continued to be free from illness.
3) A leader would never have leave to give up his authority during a time of crisis (aka when his people needed him the most). After further thought on the matter, I do not think the ability to give up one's life was exclusive to the King. It appears that any of the
Numenorean blood should have been able to do so.
Denethor's authority required him to take charge and PROTECT his people when they were under attack. Denethor's authority came from being the Steward to the King and to act in his stead; hence, he could not choose to commit suicide. In other words, had he defeated
Sauron and then chosen to retire himself (death), then that would have been within his authority. To abandon one's people in such a selfish and cowardly manner violated his responsibility as the
Steward of Gondor.
(All my opinions, although I think any serious research will validate my points.)