Tolkien always maintained that Melkor could only pervert life not create it. "and naught that had life of its own, nor the semblance of life could ever Melkor make since his rebellion"
(The Silmarillion, Of the Coming of the Elves, p.50). This lead to constant problems for Tolkien in explaining how the Orcs came into being. Tolkien it appears from the Silmarillion came to the conclusion that the Orcs were corrupted Elves.

"Yet it is held by the wise of Eressëa, that all those of the Quendi who came into the hands of Melkor, ere Utumno was broken were put there in prison, and by slow arts of cruelty were corrupted and enslaved; and thus did Melkor breed the hideous race of the Orcs in envy and mockery of the elves," (Ibid)

Their origin being corrupted Elves, and comments made by Shagrat in the Lord of the Rings, have lead to questions about the whether Orcs are in fact immortal. There is a comment made by Gorbag when he was chatting with Shagrat in the tunnels under Cirith Ungol that is of interest:

"there?s someone loose hereabouts as is more dangerous than any other damned rebel that has walked since the bad old times, since the Great Siege." (The Two Towers, The Choices of Master Samwise)

Many believe that the way Gorbag says this, suggests he himself was present at the great siege in question. The Great siege some have said refers to the siege of Angband, which was about seven thousand years ago. However I believe that it in fact refers to the siege of Barad-dûr at the end of the second age that lasted seven years. This however was still over three thousand years before the War of the Ring. It is perfectly possible though that Gorbag wasn?t there at all, such a major event would no doubt have been remembered by the Orcs as a race, for although they were not well educated Gorbag, in his position, was probably well in touch with events of the past.

Elves are of course immortal and so it would be reasonable to assume that Orcs, being corrupted elves, were too. Tolkien was always seeking, it is clear from the history of Middle-earth, to make Orcs descended from men, a much easier to swallow conclusion for him. However this would not fit into his chronology of the awakening of men. So it seems clear that as soon as men appeared Morgoth used some to breed with Orcs resulting surely in shorter-lived Orcs. There is only one Orc who provides any clue as to his age, Bolg son of Azog. Azog was slain in the battle of Nanduhirion in 2799 of the Third Age (The Return of the King, Appendix B) so Bolg could have been born at the latest in 2800. Bolg led the orcs of the Misty Mountains, and was slain, in the battle of the Five Armies (2941). That makes Bolg at least 141 at the time of the battle of the Five Armies.

As can be seen in the Morgoth's Ring, Myths Transformed, Tolkien?s final view of the origin of the Orcs was that they where bred from Men. As a result of this Tolkien gives the following conclusion:

"They could be slain, and they were subject to disease; but apart from these ills they died and were not immortal, even according to the manner of the Quendi; indeed they appear to have been by nature short lived compared with the span of men of higher race such as the Edain." (Morgoth's Ring, Myths Transformed, p.418)

So this seems to be the conclusion, that Orcs were mortal and were short lived compared to the Edain. However it must be noted that Morgoth's Ring is not canonical and come will be happier to accept the Silmarillion's version ather than Tolkien's final view, and that is perfectly reasonable. Now we are given an idea of what is accounted as short lived by the example of Gimilkhâd. "Now Gimilkhâd died two years before his two hundredth year (which was accounted an early death for one of Elros' line even in its waning)," (The Silmarillion, Akallabêth, p.269). This would make Bolg's age acceptable although it must be noted that Elros' obviously had elvish blood in it.

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