I'm having a moment...
Is there a guideline anywhere to determine the Astrological Age? My mind says there is a "listen to the music of the spheres" somewhere but I can't quite remember where. Also it might not be sensitive enough to detect such a slowly changing quantitiy as according to teh official histories the Order of Hermes has only encountered the age of Pisces.
Bob
Astrological Age is not something made up for Ars Magica, but a real world idea.
You could do far worse than reading the Wikipedia article on it: Astrological age - Wikipedia
The most relevant parts for Ars Magica is that the Age of Aries ended around 1AD (plus/minus a century or so), at which point the Age of Pisces started. The Age of Pisces is supposed to have ended and the Age of Aquarius started sometime in the 20th century.
Oh I know that, so they can look by using Artes Liberales(astronomy) to determine the age. I was wondering if there was a corresponding spell guidline? I am doing something in my campaign where the world west of iceland might not be in the same astrological age as East of iceland (to be precise the East has been stuck mythically in the Age of Aries and hasn't managed to transition). I don't want to proceed down soem rabbit holes if there is a published guidleine somewhere that could contradict my assertions too easily.
Bob
There are a bunch of spells and spell guidelines for determining astrological time in TM:RE. Nothing specific about Astrological Age, but that ought to be part of it.
Since Astrological Age (and Astrological anything else) is determined by the stars, I do not understand how part of the Earth could be in a different Astrological Age than another since the stars would be the same, but that is luckily not my problem to handle.
Generally astrological information is detected through InVi since the stars influence magic. Determining the astrological age would be pointless because it changes so slowly, it would be like investing an InTe spell to determine what hemisphere you are in.
yep this is I hope something that does not exist easily, until the players investigate for themsleves.
Detecting it shouldn't be difficult, but it's largely irrelevant on a human level.
Though we get evidence that what age you are in influences how magic works, and the change of ages seems to have caused the Cessation of Oracles, the decline of the Mercurians (and probably Magoi), so the age itself probably has a large effect on the fluid vis (what magi use for spellcasting), and it might flavor very old raw vis if a magus cared to investigate.
They just need raw vis that's over 1200 years old... 
I would think any competent practitioner of Artes Liberales (and thus astronomy/astrology) could tell you the current astrological age quite readily. Also from Mysteries, pg 51, there is an Intellego Vim base guideline of 5 for detecting the current astrological time, and/or other astrological conditions. This makes the astrological age I think rather trivial information for educated people and Magi either one.
Just to clarify what precisely do you mean by Astrological Age OP?
I'm curious. How would you see that work in practice? How would "old vis" differ from new vis?
I like the idea. I think it would just "feel" different. The difficulty now is to find soemwhere that the Vis could plausibly have been left for that time. I'm playing in Icland (using the 4th ed Land of Ice and Fire book as inspiration) so an Egyptian connection is obvious... Thinking as i type, you could even get Taurean Vis as well as Arean Vis from Egypt to compare with what should be Piscean Vis.... Hmm..
Bob
Maybe you'd like to give a look at the InVi guidelines for the Guernicus spells on HoH:TL. They have things to detect very subtle variations in spells and spell traces.
Humm... you could also think of spells cast in the age of Aries, or items enchanted in Aries, that are still active today.
Also, maybe they work differently? Parma might not be completely useful against effects from other ages?
An intriguing possibility would be vis from the age of aeries that belongs to a realm which does not seem to exist anymore.
Perhaps some sort of realm of chaos or chimeric realm... or a realm of the djinn which shattered at some point in the past and was absorbed into magic and faerie.
Hermetic magic is supposedly 'sealed' from the influence of astrological events, that's why a lot of the Astrological Mysteries are mysteries rather than part and parcel of hermetic magic (though Hermetic Magic still uses astrological events for durations, naturally).
Enchanted objects might fail over the change of ages, or they might not, it really depends on how much the influence of the stars was involved in the enchantments in the first place. Ancient periapt enchantments probably don't work, while chthonic ones wouldn't care about the passing of the ages.
Really old vis might not be very different from modern raw vis, but it might be useful to fuel old enchantment or rituals that don't work anymore because the ages have changed. For example, vis from the previous age might be used to fuel those Mercurian rituals that don't work anymore because it has the 'right' magic. Old vis might have odd effects on modern magic; seekers that discover those old vis caches probably just transfer the vis (making it 'new vis' once more) without thinking, or keep the objects for study rather than using the vis, and thus never really discovering the rare uses of this slightly different, increasingly scarce vis.
Old vis might be more useful for summoning/theurgy/necromancy, as that seemed to be a more important aspect of older forms of magic and one that Hermetic theory struggles a bit with, possibly because it's a product of its Astrological Age. Consider it like Dedicated Vis where if you use it for certain purposes 1 pawn counts as 2...