Two points raised, here...
First, the Borg. Originally, in "Q Who?", they were a humanoid race that had been developing their technological interconnectedness for "thousands of centuries". They were speciesist. Guinan and Q both discussed it: "They're not interested in you. You're nothing to them. They're only interested in your ship, in its technology. They've identified it as something they can consume." "That's not hoe the Borg work. They don't assimilate individuals -- only civilizations. When they come, they'll come in force. They don't do anything piecemeal." And so on like that. The gist was that they descended en masse on a technological race's world(s), stripped every city and artifact, disposed of the extraneous biomass (people), and incorporated whatever new technological tricks that race had into their repertoire.
But then in their very next appearance, "Best of Both Worlds", suddenly they sent a single ship to assimilate a single person to "facilitate" the assimilation of Humanity (and then presumably the rest of the Federation, and so on from there), and were suddenly interested in the biological as well as technological distinctiveness of other species. In further appearances, they continued the decline from relentless toward brainless. They scared the hell out of me in that first episode, were still intimidating in BoBW, and by the time First Contact came out they had become vaguely slimy, somewhat sexual (nanoprobe injectors, the Queen, hints from VOY "Unity"), and ignoring us because we posed no threat had devolved to bumbling around obeying their programming like like living zombies.
We did not originally see other species represented because there were none. The Borg bioform was, in Star Wars terms, a near-Human species. It wasn't until the entire concept had transmogrified a few times (First Contact and the Borg arc of Voyager) that we saw other species -- Cardassians, Klingons, Romulans, Bolians, and other races thus far unknown...
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As for "older races", we saw the Progenitor species in TNG's "The Chase" who seeded the primordial oceans on many Alpha Quadrant planets around 4 billion years ago. But in general... For as long as this galaxy's been around, for the number of supernova explosions needed to fuse the heavier elements needed for organic life, for the time needed for those atoms and molecules to drift through the galactic arms and accumulate... Our sun is, by reasoned conjecture, probably a third-generation star. It and its planetary system have a decent amound of stuff heavier than Hydrogen and Helium (comparatively speaking). Given all that, I've often speculated on whether we might be the first sapient species to evolve to this point in this galaxy. And I've seen a few s/f authors follow the same thought.
Even if we're not first, other races may be up to a few thousand to a few million years older than us. There may or may not be technological plateaus. The idea of intelligent, spacefaring civilizations on the order of a billion or more years older than us I start to consider unlikely, from the sheer lack of available stuff to make them. And, to keep the Babylon 5 comparison going, those seriously old races might have so vastly out-evolved us and how we think and experience the universe that G'Kar's comparison of the Younger Races to the First Ones -- an ant asking another ant what the humanoid who just picked him up was -- is probably pretty apt. If they're aware of our existence at all, we are at least as far beneath their concerns and they have nothing to say to us nor time to spare for us.
There are hints of that Trek, too -- the hyper-evolved races who are beyond our concepts of territory and scientific pursuit and such: The Melkot, the "Golden Youth" from "Arena", the Organians, The Greek gods (of whom Apollo was the only one who still felt the need for lesser beings to worship them), and so on. Many instances of "oh, this was our home when we were like you... we visit from time to time" or "you beings are noisy -- we'll be going away now" or "come look for us again in a few centuries/millennia when you've grown up some" and like that.
--Jonah