The Mouth of Sauron is referred to as being of great age, so great he has forgotten his own name, but not a ringwraith.
How is this possible?
How does he evade the Doom of Men?
Does that imply he in fact holds a Ring of Power? (One of the Seven?)
In the chapter The Black Gate Opens Tolkien describes him as follows:
The rider was robed all in black and black was his lofty helm, yet this was no Ringwraith but a living man. The Lieutenant of the Tower of Barad-dur he was, and his name is remembered in no tale for he himself had forgotten it: "I am the Mouth of Sauron." But it is told that he was a renegade, who came of the race of those that are named the Black Numenoreans; for they established their dwellings in Middle-earth during the years of Sauron's domination, and they worshiped him, being enamored of evil knowledge. And he entered the service of the Dark Tower when it first rose again...
The Dark Tower rose again in 2951 and the final battle took place in 3018 so the Mouth of Sauron could be Aragorn's age or older. The Numenoreans were men who had been granted long life by the Valar: Aragorn was of the Numenorean line and lived to be 212.
At least one of Sauron's primary goals is to engineer his own immortality (a point discussed in works by Shippey, Rutledge, Kreeft, and practically all the "about LOTR" books). It shouldn't surprise us that this once-Numenorean could worship Sauron, be his lieutenant, and live an unnaturally long life. That may have even been his reasons for siding with Sauron in the first place. Let me explain:
No less than Faramir---well versed in ancient lore and tutored by Gandalf himself---describes ancient Numenoreans becoming "enamoured" with Sauron and practicing the "black arts."
The Men of Numenor were settled far and wide on the shores and seaward regions of the Great Lands, but for the most part they fell into evils and follies. Many became enamoured of the Darkness and the black arts . . . It is not said that evil arts were ever practised in Gondor, or that the Nameless One was ever named in honour there.
(quote from The Two Towers, "The Window in the West")
Considering that the passage where the Mouth of Sauron enters the scene explicitly suggests that he was a Black Numenorean, it is worthwhile to consider the plausibility of connecting the dots.
My own conclusions is this: The Mouth of Sauron is most likely one of the ancient Numenoreans who worshiped Sauron and practiced (or still practices) the black arts, and through magic (in the sense that Tolkien so often talked about magic, as a type of control or manipulation not all that different than our modern technology, but certainly different than real power) lived an absurdly long time, long enough to forget his name (and, in the spirit of LOTR, to lose his true identity/self---see also Gollum; orcs (changed elves); Ringwraiths; etc.).
He was possibly granted a longer life span due to his association with Sauron. I say this because the black Numenoreans were long deceased by the later stage of the third age of Middle Earth.
The mouth of sauron had numenorean blood in him so therefor had a longer life span, yes black numenoreans dwindled in the third age and merged with other men. But the mouth of sauron probably still had quite a lot of numenorean blood in him maybe not as much as the black numenorean of tye second age but still a lot. Aragorn obvious had numenorean blood in him yet he didn't live as long as someone with pure numenorean blood, I think the same may have gone for the mouth of sauron he wasn't pure numenorean but their was enough numenorean in him to live a very long time