The origin of the Borg is never made clear, though they are portrayed as having existed for hundreds of thousands of years (as attested by Guinan and the Borg Queen).
The Borg have existed for no less than a thousand years, but it's only in the past two hundred years or so that they've begun a period of increasingly-rapid expansionism. The underlying cause for this increased expansionism is unclear. It is presumably related to 'cultural' and/or technological changes.
Some anomalies exist, such as the Alpha Quadrant Ferengi having the low number of 180, which may hint at an early first contact with the Borg under unusual circumstances. Numbers do not appear to have been re-used even after the species had been assimilated. Whether the Borg had a designation for themselves is unknown.
Species 2000 name themselves Cardassian, a warp-capable species native to the Alpha Quadrant and the planet Cardassia Prime. Known to be highly xenophobic and ruthless.
However, the Vulcans are not the oldest civilization in the galaxy. When it comes to the oldest species, the Vedala are the most ancient race known to the United Federation of Planets. This species is known to be very advanced due to its ability to control time and cure mental illness.
How Was The First Borg Created? Who Made Them? What Is Their Purpose? - Explored
Who is the oldest human in Star Trek?
The oldest I'm aware of was Admiral Leonard McCoy in the first TNG episode: no time travel, no transporter shenanigans, no cryogenic sleeper ships, just a healthy lifestyle and the best medical care Starfleet could provide and he lived to be at least 137 years young.
How old is Captain Picard in Star Trek Next Generation? The character Picard was 59 when TNG debuted. Picard was deliberately fashioned as “older and wiser” as opposed to Kirk who was 34. Someone more mature and experienced - not a “space cowboy” - to command the 1000+ crew of the Federation flagship.
Species 8472: Also known as the Undine, they were a highly advanced and biologically unique species from fluidic space. Due to their vastly different biology, the Borg were unable to assimilate or adapt to them.
The Borg discover Species 8472 in the Delta Quadrant and try to assimilate its biotechnology, which is more advanced than anything the Borg have seen. The Borg quickly realize that Species 8472 is immune to assimilation and that contemporary Borg technology is no match for it.
With their superior biological technology, Species 8472 drove the Borg back and launched a counter-invasion of the galaxy, annihilating large numbers of Borg drones, ships and even planets.
The Borg Homeworld is Earth. That is why the Borg have such a hardon for Earth that they put resources into capturing Earth on the other side of the galaxy and assimilating humanity rather than continuing their geometric expansion out from the Delta Quadrant.
The society of species 1 consisted of female drones and a queen for each colony. There were males, but they were only used for mating, much like ants on earth. Through millions of years, species 1 developed a collective consciousness on the colony level, with each colony having its own.
Could "Q" or a "Founder" be assimilated by the Borg? Yeah, maybe. There seem to be some species like species 8472 though, that are just incompatible with assimilation, but for the most part they can usually find a way to assimilate many species.
As far back as billions of years ago, one of the earliest incarnations of the Borg may have arisen, presumably one of the child races of the Progenitors (a.k.a. Preservers). These cybernetic beings fought with their creators, eventually destroying them like the Klingons would slay their gods.
The Borg would become an unstoppable force, able to assimilate or simply change the galaxy to fulfill their vision of perfection. In addition to gaining all the knowledge of a particular Q, having their consciousness uploaded to the Borg collective, they would also potentially gain the god-like power of a Q.
Q. The Q are potentially the least likely to get assimilated, for not just one reason but two. Firstly, their near omnipotence and god-like powers would make it almost impossible for a Borg, or even all Borg working together, to capture a Q.
As far as I know, the "Bad Borg" hail from the Delta Quadrant, and though we don't know just how old that collective is, it is ancient. The "Good Borg", Jurati's Borg, were established in the Alpha Quadrant in the 21st century, when the queen from the alternate timeline and Jurati merged.
As far as age of the characters go, it would be Kirk as “the youngest man ever to become captain in Starfleet”, with Picard nominally being the oldest by screen appearance.
In the fictional world of Star Trek, there seems to be some confusion about Jonathan Archer's lifespan. Archer's fictional “biography” says that he was born in 2112 and died in 2245, which would make him 133 years old at the time of his death.
