Thorongil

[Lily Bracegirdle]

To expand on what Silver Dragon said, Denethor knew Isildur's heir was coming to Gondor to claim the Kingship -- probably via the palantir. In the Pyre of Denethor, after Denethor reveals he's been peeking into the palantir, he says:

Do I not know thee, Mithrandir? Thy hope is to rule in my stead, to stand behind every throne, north, south, or west. ... So! With the left hand thou wouldst use me ... as a shield against Mordor, and with the right bring up this Ranger of the North to supplant me. ... I will not step down to be the dotard chamberlain of an upstart. Even were his claim proved to me, still he comes but of the line of Isildur. I will not bow to such a one, last of a ragged house long bereft of lordship and dignity.

Even though he doesn't explicitly say he knows Aragorn is Thorongil, this is implied in Appendix A:

Therefore, later, when all was made clear, many believed that Denethor ... had discovered who this stranger Thorongil in truth was, and suspected that he and Mithrandir designed to supplant him.

So, even though Denethor was dead by the time Aragorn entered Minas Tirith, Denethor had "recognized" him.

I think Theoden probably didn't recognize Aragorn for the reasons already stated. .... Plus, he just wasn't as with it as Denethor.

-Lily

[Orophin]

Theoden was pretty young when Thorongil aka Aragorn fought with his father. It is possible that he did not recognize him for a number of reasons. One may be that he simply doesn't remember him because it was so long ago. Two maybe he remembers the person but doesn't connect them because Aragorn lokks differnt from traveling in the wild so long. Third maybe he does know it and he just doesn't say anthing for who knows what reasons.

[Voralphion]

Or, perhaps Theoden never met Aragorn when he was there before as Thorongil. Theoden was only very young (11 I think) and so may not have had anything to do with matters of war and so wouldn't have met him.

[HerenIstarion]

Aragorn was disguised. Yes, as simple as that.

Tale of Years

2957-80 Aragorn undertakes his great journeys and errantries. As Thorongil he serves in disguise both Thengel of Rohan and Ecthelion II of Gondor.

and, though Theoden did not recognize him, that is not because he haven't seen him before:

LoTR, Book III, Chapter II

'I do not think your law was made for such a chance,' said Aragorn. 'Nor indeed am I a stranger; for I have been in this land before, more than once, and ridden with the host of the Rohirrim, though under other name and in other guise. You I have not seen before, for you are young, but I have spoken with Éomund your father, and with Théoden son of Thengel.

[symestreem]

This brings up the question of how Aragorn would disguise himself. Obviously, colored contacts are out, but grey eyes are not uncommon. Neither is dark hair, although I suppose Aragorn could have used some sort of organic dye to change his hair color. So what would Aragorn change about himself, besides his attire? He couldn't make himself taller or shorter, or give himself a humpback, because that would impede him in battle. So how would Aragorn change his features?
It is possible, even in this world, to change how people see your features (no, I am not smoking anything, but it's very hard to explain) especially if you have distinctive eyes. Aragorn's were "A pair of keen grey eyes." Perhaps this is what he did?
Also, do you think he would have made himself as bland as possible, or as distinctive as possible, so that if he returned to Rohan and Gondor, he would not match the definite image of Thorongil that people had?
Or perhaps Aragorn used some sort of elf-trick/strength of mind to disguise himself?
Or maybe... *pictures Aragorn in a trench coat and dark glasses*

[Olorin_TLA]

Just different clothes. Who's going to recognise him? There's no media to spread his face around, after all...a disguise for Aragorn would simply have been the change or name, and different clothing. I'm guessing fancy armour was available sometimes since he was a big soldier for Rohan and Gondor, and a guy called Eagle of the Star would probabaly wear nicer gear than a Ranger's travelstained kit. But nothing complex at all...he didn't need to peel off a latex mask every night before sleeping, or anything like that.

Besides, several decades, even to someone who ages physically as slowly as he does in adulthood, is disguise enough. Just see photos of your grandparents in their youth, or at the age when your parents were kids or teens. (Makes you think! )

I think he just changed his accent. He'd been to the far south, and lived in the
north. So he had probably heard a billion different languages.