Just out of curiosity:
Of all the 'official' Magi characters (those who have had stats in an Ars Magica book) which one has the highest score in any art? Can be from any edition. Please note which book, which character, and which art.
I hate arguing from assumptions so I'm throwing this open to the community here... mainly cause you (collectively) have more ArM books than I do!
IIRC it should be Caithlin Suil Uine ex Merinitae. Lion of the North (Loch Leglean tribunal book). From the Cun Clach Covenant.
Imaginem 36 (Imagonem at the time of publication). The thing about her is noit this n umber that is not THAT spectacular, but the fact that she has 11 arts above level 30 and her other 4 arts are all high 20's. 327 years of age. Basically a strong-strong fae blooded merinita. More a powerful faerie with hermetic training and free will than a "proper" human being
Remember that there's a whole book of magi in the queue...
Of course, she is also 3 editions old and basically impossible under the current rules.
The core rules (page 32, Magus Only - After Apprenticeship) have pretty clear guidelines for approximating how high Arts should be relative to age. It suggests that 120 years after apprenticeship is the longest a magus is likely to live before Final Twilight and at that age, a magus would likely have a maximum Art score of 40. In my experience, that's pretty reasonable.
In the current rule, i think its Phillipus Niger from Guardian of the Forest. Perdo 35+3. But he has a reputation of 5 or 6 "master of Perdo".
I'd suggest the genius locus of the Cave of Twisting Shadows effectively does, but doesn't notice because it is dissociative.
She was basically impossible to make under 3rd edition rules, for that matter.
It states but why? Magi who use magic more rarely or have botch dice reducing abilities can live a far longer life. I think even 170-180 years life after apprenticeship is available.
I believe the above was based on 'expected' practice, and was mainly there to set a power level for the setting. As such, it is arbitrary. The calculation is founded on monitoring Warping Points, under 'reasonable' assumptions, and seeing when Final Twilight is achieved. It neglects the fact that Final Twilight is not achieved at any Warping score by itself, however - it takes a botch or something similar.
By taking care to avoid any stressful use of magic, magi can live to many, many centuries. Options opened up in supplements further allow various ways to achieve 'immortality', allowing other paths to old age and power. The maximum age is therefore a matter of fiat. The levels set above seem reasonable to me as the power level for the setting. (I even prefer a lower powered setting, but that requires changing the core rules.)
Twilight is still a limiting factor and options from supplements are optional. (Ok, I'm not a fan of supplements.)
How is twilight a limiting factor for those who never take any risks? Barring botches, twilight is virtually insignificant.
No botches? That means never using stress dice, so no spontaneous or ritual magic or magic on distracting places. That is very uncommon!
Thijs:
Bookworms, elixir specialists and lazy Jerbitons rarely need to use magic. Not the powerful magi will live the longest life - except they make something with mysteries (I suppose).
By the rulebook magic theory is a strong limiting factor to make elixirs or rituals. As I calculated for myself it is more limiting than the lab total.
The age of magi appears only in the case when we make npc magi for a whole tribunal. What age could the praeco have 160 or 250? And what about the other elders?
Why on earth would that be uncommon for an archmage? Player character tend to go on adventures, NPC archmages tend to stay in their tower engaged in their esoteric pursuits. He can stay in his tower and cast whatever cantrips he can cast spontaneously without seeing the dice, and Mastered spells (ritual ones too) without ever seeing a botch dice. In the meantime, he can continue to study his favorite Art from raw vis in his Safety-maximized laboratory, invent and more spells, practice to Master them, enchant more items, read more tractatus, and so on. There is plenty to do that doesn't involve putting yourself in stressful situations; that's what younger, PC, characters are for.
If you want to say arbitrarily this won't work so all magi die on a botch shortly after they reach Warping 10 - that's your prerogative. That's also the tacit, implied, idea in the core book. But it isn't at all unreasonable for a powerful magus to distance himself from such sources of danger. (Even if he does, note that a Twilight episode does not automatically follow - he'll need to botch, and to fail with at least two dice...)
The key issue here is not the rare use of magic, but rather the careful use of it. A magus who Masters all his Formulaic and Ritual spells, places an emphasis on his laboratory's Safety, and takes care to avoid stressful situations and non-trivial spontanous spells will be able to pursue nearly any archetype. Whether a Verditius crafting magic items or a Tytalus overseeing a vast network of spies and intrigue, most roles can be done carefully.
Did you include the ability to increase the age with more Raw Vis? I vaguely remember that pushed the maximum lifespan to many centuries.
Yes, that is a problem. Especially if you do want to include the various 'immortal' magi out there.
That means never using stress dice where you'll be subject to more than one botch die, not never using a stress die. It takes a double botch to trigger a twilight episode.
Non fatiguing spont spells use no dice at all, as an archmage these actually become useful.
Cautious sorcerer, Hermetic theurgy, Flawless formulaic magic, Ars notoria and other virtues are available to cut down the number of botch dice a character rolls. as a character ages they will likely get more spell masteries.
Ritual spells don't require a stress die if they are cast in non stressful circumstances. (p. 81, this is specified for forumulaic magic in the right hand column and then ritual spells are said to be cast in the same way in the center column, yes it's not very clear )
I don't think that it is particularly uncommon. One doesn't get to be an old magus by casting stress spells in hostile auras.
edit: I guess I'm not the first to chime in with this line of thought, sorry.
Caitlin Suil Uine ex Merinitae is *totally reproduceable under 5th edition rules, keeping in mind the potential use of the Faerie Magic Mysteries and Becoming. In fact, I will make that a hobby project 
. Recreate Caitlun. I have already recreated Carles of Jerbiton from Iberia and Erat Caecus of Tytalus. I think i could do it.
And I happen to particularly enjoy and appreciate supplaments. I don't see it as rules creep as severly as others. Rather, I see it as having pieces of the game returned to us after bbeing taken away for the new edition. Example: I have the original Mysteries, got it many years ago when it first came out. When 5th edition came out, there was such a radical change etween editions that I had to wait a few years for TMRE before I could use Mysteries again. I had to fake it with patches I invented for important NPCs and elder PC's.
And the core rules, the reference as to how long an average magus lives after apprenticeship, I think that estimate is way to conservative. A typical magus should live much longer than that. They certainly seem capable of that in the actual in-game examples I have seen.
Hmm, I might be wrong indeed. But still I expect these aged wizards to be fairly rare. (they need to be both cautious and lucky, because they should never throw too many 1's on an aging roll).
But this topic was mainly about theoretical scores, so I will crawl back in lurking position 
The problem is making a new longevity potion when you are old.
Say, you have a CrCo Lab Total of 50 and make a longevity potion at age 35. This is great, until you reach age 100. After age 100 the penalty from your age is greater than your bonus from your longevity potion. So, you need a new longevity potion made with a Lab Total greater than 50, otherwise you will be very lucky to make it to age 150.
Of course, now that you are age 100 your lab total has increased too. So, you get a new longevity potion made with a CrCo Lab Total of 75. This is good until you get to age 150. So, now you need Lab Total greater than 75 for your Longevity Potion to be of any use...and so on.
Basically, unless you become immortal via some kind of Mystery (the rarity of this is probably highly saga dependent) you only have a few decades after your CrCo Lab Total becomes ineffective...and once you are that age whenever you undergo an Aging Crisis there will be a chance of death.
Or simply you hire a longevity specialist right at the first time with lab total of 100-150.