tl;dr: Max out bonuses on aging rolls and delay longevity ritual as long as you can. Use every advantage to reduce magical botch dice to lower Warping Points and the chance of triggering Twilight. With that, age 300 is easily achieved, 350 is probable, and 400 is possible. At the extreme, a magus could last into his 600s before his LR gave out, assuming he could avoid triggering Twilight.
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So I got to wondering just how old you could get a magus. (Theoretically, of course. No saga is ever going to go as long enough for this analysis to matter.) To get a magus up in years, you have to battle the twin problems of Decrepitude and Warping. I ended up designing an "ideal" long-lived magus and then ran some numbers. Here's what I got.
Let's start with Virtues, because that can make all the difference. I opted for the following:
Strong Faerie Blood: This gives a critical +3 to Aging checks.
Mild Aging: Another +1 to Aging checks. The +3 to Aging crises is nice, though in the end it probably won't matter.
Affinity with Creo and Corpus: This helps him get a high-level longevity ritual when the time comes.
Affinity with Magic Theory and Puissant Magic Theory: This gets his MT up as high as possible so that he can use as much vis as possible. (This will allow him to make a LR at a high age when his Cr, Co, and MT will be high.)
Careful Sorcerer: Lowering his botch dice when casting will significantly drop the number of Warping Points he gets.
Flawless Magic: By mastering all spells, he can actually reduce the number of botch dice on his formulaic spells to zero.
Minor Magical Focus (longevity rituals): This will significantly increaase the potentcy of the LR he can get.
For those of you playing at home, you may have noticed that this gives me 13 points of Virtues. Not to worry. We'll just make him an Ex Misc magus with Strong Faerie Blood as his supernatural virtue. (Legit according to HOH:S.) That pays for SFB, leaving the accepted 10 Virtue points spent. Technically, this would allow Careful Sorcerer or his mMF to be taken as the Ex Misc Hermetic Virtue, freeing him up for another regular virtue. But that might be pushing things too far. We could take Mastered Spells as his hermetic Ex Misc virtue, just to give us a head start on building up those spell masteries. And that could fit with the concept of a faerie-blooded magus for whom magic comes easy.
Flaws are largely unimportant to this analysis. So long as he doesn't take something like Difficult Longevity Ritual or Poor Living Conditions, he should he okay.
For Characteristics, let's just make sure that he has a good Int and a good Sta. Those are what matter in Aging and Twilight.
For Attributes, he'll start with good Creo, Corpus, and Magic Theory. He'll also have some points in Concentration, Vim, and Mentem, with the requisite 5 points in Parma Magica.
Now, onto advancement. Obviously, the character has to devote a lot of time to learning Cr, Co, and MT. But what else can we do to help him along?
First thing is to get a familiar as soon as possible. (CrCo, of course) It doesn't matter if the familiar bond is low to begin with. (We can always build it up later.) Let's assume that he can get a bond value of 30 by the time he reaches the age of 30. That doesn't seem unreasonable since his Cr, Co, and MT will start high. At this stage, we put it all into the gold cord. That gives him another -3 to botch dice with magic, bringing his total for most spells up to -7, and -8 for those spells that he's built up the mastery to level 2. Moreover, since at least one of these dice is from mastery, that means the botch dice can go down to zero. Given the situations I've seen, it's rare to have 9 or more botch dice on a roll. So, if he limits himself to formulaic spells as much as he can (the chance of this varies by saga), the magus can almost eliminate the accrual of Warping Points.
We'll need to rebond the familiar before he's 50, of course, to get some bronze cord action. Let's be conservative and say that we only get to a 45 bond value by age 50. That still gives a +2 bronze cord with a corresponding bonus to Aging checks.
Once he turns 50, we start making Aging checks. But he doesn't get a longevity ritual yet because of the Warping. We'll assume that by now he's managed to get into an Autumn covenant, so his base living conditions are +2. For the sake of argument, let's also say that he picks a covenant that has a +1 living condition. This means that his Aging checks are at +9, which is pretty good. Even with the minus for current age, that still puts him in a pretty safe place. For most Aging rolls he'll at worst get an increase in apparent age. Only rarely will he get an Aging Point. And the chances of getting an Aging Crisis are very slim. We should easily be able to drag this out until he passes the century mark, probably a few decades past that, especially since during this time he strengthens his familiar bond again to raise his bronze cord another point or two.
Meanwhile, he's been doing more than just raising his Cr, Co, and MT during the past 80-100 years. Our magus has also joined the Order of the Green Cockerel and started initiating in its mysteries. He should get to the second tier reasonably quickly, we hope, which gets him Unaging and stops what Aging Points he might get from harming him too much. He also gets Hermetic Alchemy, which helps him start to accumulate the vis he'll need for future longevity rituals. In time, it will also get him Lesser Elixir (which helps with LRs) and Philosophic Alchemy (which further ramps up the accumpulation of vis). We'll set aside the Great Elixir for now, since I'm trying to find out how long you can stay alive without trying to become immortal. (Let's just assume that philosophically he doesn't want to make the sacrifice of humanity required to become immortal.)
We'll also want him to pick up some Cheating the Reaper types of spells (designed for him so they cause no Warping) for when his inevitable aging crisis hits. After all, it only takes 5 Aging Points to get there. He'll probably get the first crisis around about age 100-120. Let's say it's 120 so we have the worst possible crisis. He's going to roll a simple die +12 (for age) +1 (for Decrepitude). That's a 50/50 chance of a Terminal Illness. But by this age he has vis and a CrCo 40 spell he can cast to cure himself. (I couldn't find anything in the rules to say that you couldn't cast a Cheating the Reaper spell to save yourself.)
The aging crisis is probably the hint that it's getting time to prepare a longevity ritual. He might be able to last a decade beyond this point. But he has to consider the chance of rolling a 13 on his Aging roll (which triggers the next crisis). Once he gets down to a stress die +4, he'll have an Aging Crisis on the roll of a 9. That's a one-in-ten chance of a crisis, which is just too much to allow. He finally prepares himself a longevity ritual.
By this point, of course, his Cr, Co, and MT are relatively high, meaning that he can create a pretty darn good LR. Let's say he's 130, so he has to spend 26 vis for the ritual. All he needs is an 11+2 in MT to do that, which is easily doable by age 130 with Affintiy for MT. This LR will definitely see him through for a long time. But even so, we'll want to push things to the limit. We'll want to monitor his age and his MT and renew the LR as late as we can based on what vis he can spend. Moreover, we want to give him some time to make a good lesser elixir to add to it. That can give a -10 or more modifier to the LR, which could potentially buy him another century of life.
So, the longevity ritual is in place, and it's a good one. Now the issue is Warping. He starts to accumulate at least 1 Warping Point a year, no matter what he does. By now he might well have a +4 gold cord and Mastery 2 in all of his important formulaic spells (with Mastery 3 or more in some of the really important ones). That's -9 botch dice, potentially down to zero. If he manages to restrict his use of spontaneous spells, he will have kept his non-LR WP gain down to nearly zero.
As a further note, he can also make sure that he enchants a talisman and some various magic items. (He has the vis by now from Philosophical Alchemy, even with saving for LRs.) This allows him to use many of his spells from items instead of casting, which means that in bad situations (e.g., in non-magical Auras) he can use items and avoid lots of botch dice. We'll also have him avoid being subject to high-magnitude spells against his will (a decent Parma Magica should suffice for that), and avoid being under the effects of spells for a long time (easily enough done).
Normally the rules assume 2 WP/year for older magi (which I assume includes the 1/year for the LR). With this build, we can expect to drop that to maybe 1 WP/4 years, or even less. Even so, those WPs will start to accumulate, if nothing else from the LR. But all hope is not lost. Even when the magus gets to a high Warping Score, that's not the end. Yes, once your Warping Score is 10 (275 WP) you're likely to hit Final Twilight anytime you enter Twilight. But we have to consider the chances of entering Twilight at all. First, it requires getting two WPs at once. With -9 botch dice, that's going to be rare. Then there's Twilight Avoidance. That's:
Stamina + Concentration + Vim Form Bonus + stress die vs. Warping Score + Number of Warping Points gained + Enigmatic Wisdom + local aura + stress die (no botch)
To simplify things, let's assume he's upped his Stamina to +5 with CrCo spells (his CrCo should be pretty high, and he'll have the vis to spare), he's pumped some points into Concentration to get it up pretty high (say an 10 with a specialization in Twilight avoidance), a Vim bonus of +6, no Enigmatic Wisdom, a local Aura of 5, and 2 WP gained. With 10 WS, that makes the roll:
22 + stress die vs. 17 + stress die (no botch)
That's not ideal, but on the off chance that he gets enough WP to check for Twilight, he has a better than average chance of avoiding it. He can do better if he ups his Concentration or Vim even more, though he'll do worse if he's in a higher Aura. Not something I'd bet the farm on, but he won't have any chance by this point, so he'll have to accept the risk.
Once he does enter Twilight, odds are he's not going to comprehend it, given the rolls he has to make. So he's in for the whole duration. That means that until he reaches a Warping Score of 10 he's relatively safe. (He needs to botch the comprehension roll at a WS under 9 to go into Final Twilight.)
Depending upon the number of WPs he gets over the years, he'll probably top 300 before he reaches a Warping Score of 10. He might even hit 400 or more if he's very careful. But at that point he's a vial of nitroglycerine, just waiting for a shock to set it off.
This being Ars Magica, I ran some numbers to see what they looked like. I took a conservative estimate and said he'd get 1 WP every four years. Given that the build is designed to avoid WP, that seemed more than fair. Under that assumption, he gets to a Warping Score of 9 at 288, and a Warping Score of 10 at 328. Obviously, if he gets fewer WP per year, he lasts longer before he gets to a WS of 10. Just for grins and giggles I ran the numbers for him getting 0 WP/year from sources other than his LR (the ideal to strive for). That's not likely, I know. But it does represent the best he could hope for. That takes him to age 404 before he gets to a WS of 10. At that point he's a ticking time bomb, just waiting out the clock. But he still has to get 2 WP before he triggers potential Twilight. And with -10 (or more) to his botch dice by this point, he's unlikley to roll more than one botch die, even with spontaneous spells. So he might last a good while beyond that if he's careful. His LR isn't a problem, since that'll last him until past 600. That's the point that his aging penalties start to get too high for his LR to cope with.
Now, I understand that you can't always control the WP you get. And there are spells that he'd be particularly vulneraable to that give WPs. He also could roll really badly on any of his aging rolls, and trigger a crisis, even with a really good LR. But I did this as an intellectual exercise to see just how long it was possible for an ideally suited magus to survive without immortality and under ideal circumstances. Based on my analysis, it seems to me that 300 is easily achieved, 350 is probable, and 400 is possible. After that, things get really sketchy. But in theory, with everything going just right, he could last into his 600s before the LR gave out.
Of course, this assumes all goes right with a magus designed to live a long time. Each advantage he has that's stripped away and each assumption dropped lowers the maximum age by a bit.