I don't think there's much in the text to support that kind of imagination.
Your imagination is wrong. Your guess is wrong. And AH and FrankTrollman are, of course, wrong.
If only there were books that contained histories of those nations' founding ...
Where there not nations of IE's in Earthdawn? I don't actually know. Which is why I'm asking. But if there were, I can't imagine they all died out between the 4th and 6th World.
No, there weren't. The handful of IE's we know about -- and there really aren't many of them -- were also the exception, not the rule, in
Earthdawn (and many were high-Circle adventurers back then, royalty, etc). There were elven nations, but not IE nations (just like not every elf in the Tirs is immortal, in
SR).
And if UGE happened in 2011 and Tir Tairngire was founded in 2035, that'd put the oldest population at 24 but most of the population much younger than that. This is all based off of second hand information, I haven't read the Tir Tairngire source book, as a disclaimer. But, I can't imagine that a handful of 18-24 are going to hold off the SSC, who'd have a larger military presence. However, with IE support, I could see the tables easily turning.
The three ways to reconcile the age thing are to either accept that even the awesome first-and-second edition writers could grossly oversimplify stuff, get psyched up, and make mistakes,
or to genuinely believe the Tirs were formed with armies of child soldiers
or to imagine dozens of thousands of immortals, themselves all thousands upon thousands of years old, mobilizing as a modern army (after being secretive for centuries upon centuries), in order to carve out Oregon as a nation (and who have done nothing noteworthy since then, and have all just faded back into modern society and pretended not to be thousands of years old, except for that half-dozen of them that shadowrunners share conspiracy theories about).
Of the three, explanations, I believe Occam's Razor generally swings towards the first as the most plausible.