[Pam Korda, Leigh Butler, Craig Moe]
Jain seems to have a cult following among the Jordanites. "Jain lives!" they proclaim. So, as promised, here is a list of all the suspects in the "who is Jain in disguise" contest.
In [Guide: 15, The World after the Breaking, 147], we have something about the last time Jain was seen alive:
"No one knows if anything lies north of the Blasted Lands other than the frozen ice of the northern ocean. Jain Farstrider was said to have willingly traveled there; however, whatever knowledge he gained was lost when he vanished within its trackless depths."
From [TEOTW: Glossary, entry "Farstrider, Jain", 663]:
"A hero of the northern lands who journeyed to many lands and had many adventures; the author of several books, as well as being the subject of books and stories. He vanished in 981 NE, after returning from a trip into the Great Blight which some say had taken him all the way to Shayol Ghul."
From [TEOTW: 47, More Tales of the Wheel, 594-595]:
Lord Agelmar is telling the story of the fall of Malkier: "When Cowin Fairheart's treachery was revealed and he was taken by young Jain Charin - already called Jain Farstrider..."
Malkier fell a little less than 50 years before TEOTW (say 45-50 years), according to Agelmar. At the time, Jain was a young man, say between 17 and 20. Thus, at the start of TEOTW, Jain would be 62-70 years old--pretty long in the tooth.
From [TEOTW: 51, Against the Shadow, 638]:
Ba'alzamon, to Rand: "'Jain Farstrider, a hero... whom I painted like a fool and sent to the Ogier thinking he was free of me.'"
This must refer to Loial's story in [TEOTW: 42, Remembrance of Dreams, 532] about the man who came to Stedding Shangtai shortly after the Aiel War, on the verge of death, but then vanished again after recovering: "'He said the Dark One intended to blind the Eye of the World, and slay the Great Serpent, kill time itself.'"
The Guide notwithstanding, then, the Ogier in Stedding Shangtai were apparently the last people to see Jain Farstrider alive before he vanished for good - until now, perhaps.
WH finally gives us a non-loony candidate for Jain's identity: Noal Charin.
Noal fits what we know of Jain to a "T." He's the right age, he's widely-traveled (or at least seems to be), he has the same last name as Jain, and he's good at telling stories of his travels. Furthermore, he is interested in Darkfriends, matching up with Jain F.'s association with the Shadow. Noal also seems to be intimately familiar with Farstrider and the circumstances of his life. [KOD 6]
People have objected to this, on the grounds that it's rather contrived to think Jain Farstrider would attempt to hide himself by only changing his first name and not his last. It's also been suggested that Noal might be a relative of Jain, perhaps a brother, and not Jain himself. Noal corroborates this theory by claiming Jain was his cousin when asked about his knowledge of the world and tale-spinning.
Note, however, that Jain was probably mind-fucked pretty well by Ishy twenty years ago. If Noal is the barrel man from ACOS, as seems clear (see section 2.2.3), we know that he's got some memory problems (and if he is additionally Graendal's old man, her attentions undoubtedly would not have helped his mental state either). However, it seems clear in his conversations with Mat that he is deliberately hiding his true identity, rather than simply being unaware of who he his.
As for the "relative of Jain" idea, we've been anticipating Jain's appearance for years. It'd be supremely lame to have his brother/uncle/nephew turn up in his place.
It seems pretty clear that Noal is Jain, but just in case, here are the (mostly loony) alternate possibilities that have been proposed: