Well, as is the case with LOST, more questions than answers in last night's season finale.
So Jacob never ages, just like Richard Alpert. And he visited all the Losties before they crashed on the island. Then he gives Hurley a guitar case. Then he "appears" to be killed by Ben and Locke -- but no one ever really dies on this island, especially someone who appears to be immortal, so I expect to see Jacob -- or at least the ghost of Jacob -- again. (Wouldn't it be funny if Jacob's last name is Marley.........?)
And Locke.... Dead and alive at the same time? His live body could be inhabited by something or someone, but I think he's just in two places at the same time. The dead Locke and the live Locke could be different ages, from two different time periods.
Juliet's probably not dead either, after having detonated a thermo-nuclear bomb with a rock. The resulting explosion probably releases some electro-magnetic radiation resulting in some weird time-shifting, in which everyone ends up in the distant past, where Jacob meets them for the first time.
Interesting how everyone seems to want to kill Jacob....... Perhaps he's the Devil, collecting souls, and the island is his hell.
Whatever the case, we have to wait until at least January of 2010 for the final season of LOST to begin. Maybe then we'll finally get some answers!