DonMoody
I'm going to go with two presumptions here:
That the immortal has huge hard drive space in their head. If it's not true, you'd get fuzzy/fade issues with old or unimportant data. . .like your 1,000 year old in 2007 has only vague recollections of the minutia of life in 1600. some Vampire fiction is written this way, with the immortal remembering about 100 years of their life, the rest only remembered via written diaries, which they read as if in the third person, having no actual recolections of the events in the diaries. Let's assume they can in fact remember a 1,000 year span like we can recall a 100 year one, so the fuzzy problem is limited.
That the immortal is immensely adaptable. . .Most old people get terms like "Set in their ways" applied to them for a reason. a minority of people truely adapt to a new world, mostly they die off, so the average population adapts to the new world. . .my parents have finally grasped e-mail, but they both wrote letters and knew how to type, both can hit record with the VCR and leave the house, but neither can program the VCR to go off at a set time regardless of how well it's explained to them. Neither has any real "gut" understanding of computers or an appreciation of 99% of the music written after 1970 either. Some old folks do adapt well, my boss is the same age as my parents, is a joyful technophile, is intimately "gut" familiar with computers, and even he falls short of "getting" conversations of a group of 40- year olds. There are exceptions that surpass him. . .
I don't have any problem with beleiving one single individual could be so exceptional as to have total recall of a lifespan measured in millenia and also so adaptable as to be able to fit in and keep changing to fit in, over and over and over again.
I find it less beleivable, but still can manage to imagine an entire race of such people.
The problem lies in mixing them with "normal" people. . .even a small ratio. . say 50,000 actual elves in the real world, who live such long, adaptable lives. . .I may be cynical, but I only see three possible scenarios.
1) The elves are in charge. An entire race of super beings, it wouldn't actually be too hard to take the long view and take over humans.
2) The elves are extinct. Humans may not be immortal, but we are the best killing machines on the earth. Ask the Cro magnons, or the Neandertals. . .they don't even exist on some small island, or in some sheltered valley. . .our ancestors utterly out competed with them and/or exterminated them. (this is also the logic behind #1, with the elves being the ones out competing or exterminating the humans.)
3) Elves and humans are seperate. For whatever reason, they keep their affairs seperated enough to not have 1 or 2 happen.
#3 is the standard fantasy world answer to the problem. . .I find it the least likely of the three, and likely only a temporary thing. (Even in Middle Earth, the elves eventually had to exit stage right.). . .it's fantasy, I can accept the least likely solution, or accept that the time frame we are looking at is the one gap in time where this weird seperated but still in contact situation exists.
But "realisticly", I don't see how the situation could last.