Hmm. I think it's arbitrary, personally, and up to whoever first discovered it or linked to it, just like the names of many geographic features on Earth. The names of places, like pretty much everything else in the English language, are not ordered or categorized logically. (For instance, the Caspian Sea, which is arguably a large saltwater lake, not a sea, because it doesn't connect to any ocean. Similarly, we could easily argue that the Gulf of Mexico should be categorized as a sea, rather than as a gulf, given the fact that it is at least as open to the Atlantic Ocean as, say, the Mediterranean Sea is. But people named them the "Caspian Sea" and the "Gulf of Mexico" - and those names stuck, even if they don't make complete sense.)
You've got Reziksehv, the pod age, yet even though it's one planet it is usually referred to not as Reziksehv but by the names of its pods (i.e. Payiferen, Negilahn, Dereno, Tetsonot.) Doubtless in the minds of most new players visiting Uru for the first time, these pods are on different worlds, because they look so different. That confusion has been present ever since Cyan Worlds released those pods one at a time - and gave each pod its own name. Gametap publicized the release of each pod individually as it appeared in MO:UL, in the same way that new ages were announced - and never once publicized "Reziksehv". So there was a tendency to assume - if you were a new player - that they were different ages. The same thing happens with our age, Earth. It gets referred to by a bunch of different names depending on where you are. There've been countless newbie players who were confused and thought that Tomahna and/or the cavern were in different worlds, and didn't realize at first that they were actually all part of Earth.
So those are examples of a single planet which is split into multiple "ages" in many players' minds - and then there's Todelmer on the other extreme, which has a planet and a moon, both referred to generally as the age of "Todelmer". The Todelmer pod is closer to Todelmer's moon than to Todelmer. But we call it the Todelmer pod anyway. Why? Because in our minds, the moon is part of Todelmer. "Todelmer" is the name of a planet-moon system. It's fairly illogical, but it happened because Cyan Worlds never gave that moon a name, so that moon, by default, is part of Todelmer and not a separate age.
Similarly, Eder Gira has a moon but we've never named that moon either or referred to it as an age. It's just thought of as part of Eder Gira. And Spire's sun - is thought of not as its own age but as part of Spire, as with pretty much every star in the sky of every age.
So, how far apart do they have to be in time or space before they're a separate age? Answer: it depends on the social consensus. Whatever the fanbase - and Cyan Worlds - collectively classify as an age, is an age. And the classifications don't necessarily always make sense.