Has anyone ever calculated just how old a magus might, with the best combination of environment and luck, live to be before dying or more likely being taken by Final Twilight? Is there perhaps an answer in one of the books that I don't have (or have overlooked this info in)?
When I try to go about figuring it out I get intimidated by what seems to be a wide range of multiple variables. I use math quite a bit in my work but am far from a professional mathematician.
There isn't really a fixed maximum.
To get really old you'd need to get a really good Longevity Ritual created for you when you are young.
How good it can be depends on who creates it - how high Lab Total they can get.
Theoretically a really good longevity expert can get a Lab Total around 200 or so. (Possible but unrealistic and requires a lot of min-maxing.)
With such a good Longevity Ritual you could probably live until the age of 500.
However, use of that Longevity Ritual will warp you. Around age 200 or so you will likely have enough warping points that your next Twilight episode will be the final one. So warping is more likely to end you than age, but if you are really careful about using magic you can avoid that final twilight for a long time.
All this is fairly rough estimates though - more exact values will depend on too many variables.
275 years post LR will take a Magus to Warping 10 if they never get a point from any other source. That would put them at 310 or 325 years old if they started taking a LR at the last possible season before aging. While Warping 10 does not outright end a Magus, it puts them in the position of any gain of 2+ warping points is Final Twilight.
You need a total modifier that is at least equal to 1/10th your age to be reasonably assured of not aging (it is after all no botch Stress roll and can explode). Discounting modifiers for Living Conditions, Cords, and Virtues this means you need a lab total of at least half the age expected to live to. All the discounted modifiers reduce the needed lab total by 5 per +1. Getting a high enough lab total to support an age well north of 300 with modifiers from Living Conditions/Cords/Virtues has been shown to be possible multiple times.
So while it is possible to live 300+ years post LR, getting north of 200 years post LR is the point where Final Twilight is the biggest danger. For active Magi this is a pretty hard limit, since it effectively forces them to retire to a very passive and safe lifestyle. If they make it to this point then even the various paths of immortality such as becoming a Being of Might are mostly closed to them due to the high chance of gaining warping points and entering final twilight.
Ultimately a very long lived magus will have to build up their creo and corpus in order to increase the amount of vis they can put into a longevity ritual as they age. As such older magi are likely to be the longevity experts selling LRs to younger magi to gain the vis they need to stay alive.
As such what the LR totals are, and hence how long the bonus can be expected to keep you alive, becomes an iterative function with each generation lasting longer and developing better LR totals. Young magi who can buy a LR with a lab total of 90+ can expect to live for a very long time.
Just for reference, the oldest magi in any official ArM5 products are 197 years old. Both are Ex Misc magi. One is depicted as a decrepit wreck functionally unable to work magic, the other is a Saga-level primary antagonist at the top of her game.
Them being the oldest at the age given is also highly unlikely. Despite what any of the books might say about the average age and lifespan of a Magus, there have been multiple people who have written and run simulators since AM5 first came out. You can find old threads talking about them, along with a link to the files if you want to run it yourself. What those simulations show is that the actual "average" life expectancy of a Magus is around 150 years post gauntlet, with the longest lived 10% around 190 post gauntlet. The top 1% are into the 200's post gauntlet.
The numbers from the simulation might actually be low for some saga since the average cause of death for older Magi is aging roles, with Final Twilight being minimal. While 1 in 10 aging rolls explode and 1 in 100 double explode, in a simulation running thousands of Magi (the total that have ever been in the Order, not just the number around in 1220) your outliers will not get any explodes (or very low one) and actually bump up against the age limit of their longevity ritual. The simulator tends to top out at around a LR lab total of 100 (200 years), with additional decades coming from lifestyle and familiar cords.
The written blocks of AM5 not lining up with the actual numbers is not something new or limited just to the life expectancy of a Magus. In the case of the age of a Magus, I know Ben McFarland (EDIT: I mistakenly said Timothy Ferguson) popped up in a thread talking about aging and was surprised when he ran the numbers when adding in lifestyle, lab health, and the familiar bronze cord. From his reaction it would seem that those were not normally taken into account when writing the age of a Magus. On average those modifiers should add half a century, with the potential of a century plus.
Magic Theory only sets the limit when designing a Longevity Ritual, but does not matter when reusing a LR. When reusing an LR, the limits to vis are the same as when casting a ritual spell: Te + Fo pawns.
Magic theory provides a limit for making a new one, but creo and corpus provide a limit for the vis that must be spent to utelize one that has already been made when it fails. It doesn't really matter if you have a level 90 LR if the first time it fails you can't use enough vis to repeat it.
The type of vis which can be used is more than just Creo and Corpus. If nothing else Vim vis can also be used. There is also the option to substitute those with a TeFo that your magic is strongly associated with. Many Flambeau for example would have Ignem replace one of those three Arts.
So worst case we are looking at your score in three Arts to determine if you can use enough vis to recast a LR as the limit. Depending on your group, one of those three Arts has a high chance of being your highest one.
I was fiddling with doing up a archimage once. Strong Faerie Blood help enormously. So does the unaging virtue and the bronze cord. Unaging is particularly good as it staves off stat damage, allowing delay of the first LR to around 80-90.
After that add 250 to 275 to get max age before final twilight.
You can build a character that is immortal by using a might 1 or 2 magical being with the gift power and xp boosting virtues. No aging, no twilight, other problems.