So I guess Kled confirms that Yordles are near immortal magical beings?

I like the angle that the yordles are becoming muses or avatars of concepts, Kled just represents the stabby nature of Noxius in the most direct sense. Granted this is the OLD concept, he doesn't seem to be that modern with how much he disses the current Noxians. The same way Poppy is probably far more original Demacia than anything else.

(Also the long lived status might be related to embodying these concepts as well, with the yordles that don't HAVE a thing that they are a concept of maybe having shorter or more typical mortal lifespans.)

Now the real question, what does the other yordles represent with stuff like Teemo?

Well Poppy was already alive before Demacia was a thing. considering that Kled looks much older than her,he is likely much older than Noxus as well.

At the very least they're long lived thanks to their innate magic. No one's really truly immortal in League, they're just absurdly long lived.

I'm actually wondering how long lived Skaarl is now.

Kled's apparently been around for as long as Sion, if his taunt is anything to go by ("I liked you better when you were alive!), so I guess that supports this theory.