Say you had four Decrepitude, and are about to gain the fifth that will kill you, then cast a Year-Long MuCo spell to grant yourself youth; including the removal of age penalties and decrepitude. Can this be done?
I know it can't be permanent because of the Limit of Essential Nature. I know that even were it to be permitted, this person will instantly die when the spell ends for even a moment, as the decrepitude is still there and waits to reassert itself. I know said person will be unable to physically harm anyone with magic resistance.
By the Limit of Aging (ArM5 p.80) Hermetic magic cannot remove Decrepitude. So the spell you think about does not affect your character's Decrepitude 4 and his chances of getting the final Aging Points, as described in the Decrepitude chapter on ArM5 p.170. Once he has reached Decrepitude 5, he will die.
What if you turn into a fish or other form that wouldn't make aging rolls for centuries or suffer from having decrepitude? The effects of aging is explicitly able to be mitigated, making it's removal/suppression a matter of 'tomayto tomahto', and the mandated maintenance of Muto doesn't break the Limit of Essential Nature.
A human turned into a fish can also still be affected by Corpus spells, which extended a bit further seems to suggest that Essential Nature is still intact and therefore you will still soon die at decrepitude 5.
So, you ask, a Bjornaer's essential nature is also their heartbeast,and what if their heartbeast as longer lived than a person. I'd say that they could switch to the heartbeast form, never to return to human form again. Perhaps, to a Bjornaer, death and Twilight are the same, literally, they adopt the form of their heartbeast, where it lives out the natural existence of whatever creature it is, maybe even resetting the clock back to the animal equivalent of 35. Either way, it results in the character exiting the saga.
If a Muto magus is really desparate for a form where decrepitude cannot reach him, there are spells like (ArM5 p. 131f) Cloak of Mist or Transform to Water. Looking frantically through Isidore of Sevilla's works he might - perhaps, I don't check that now - also find a fish told to be immortal: perhaps a jellyfish. But all these are not really useful as long term forms of a magus. They just make spellcasting and labwork really troublesome, and heap Warping Points onto him.
Perhaps the magus wishes to look at ReMe 55 Exchange of the Two Minds (ArM5 p. 152) or the procedures around The Living Corpse (HP p. 96ff) with lots of Rego and some Muto instead? But most serious Muto magi out for eternal life will find rumors about The Great Elixir (TMRE p. 43f) in time, and join the great work of the Green Cockerel.
Ars Magica does handle immortality really well; if you're character is immortal your character exits the saga. The path to immortality, the journey, if you will, is what is important, not the destination. In many sagas, the longevity ritual alone makes a magus character (or other characters to whom one is given) functionally immortal, because the saga may well end prior to the point one has to deal with the effects of aging at all.
If it were a PC that became immortal, I'd use him as an NPC with a lot of guidance from the former player, or the player plays him with a lot of guidance from me as the SG on occasional stints, guest starring roles in that night's episode. The character would be changed, much as Sinclair returned to Babylon 5 in War Without End, he was different, and about to become even more different. Granted, Sinclair wasn't immortal, he was about to become immortal in many respects, though.
I think that is absolutely not implied. Why would we have all the rules about how such immortals advance (or don't) in Abilities and the like if the implication is that they leave play? Plus, multiple of the routes have you immortal before you complete them, which means the character exits the saga before completing the process to exit the saga?
Granted, mine is an inference, based on the fact that I do not want to play an unchanging character, or for whom one spends all of his time pushing for a minor change, that it effectively keeps him out of the saga for years while he's pursuing that change. So, sure, stick around and don't change, or pursue your own concerns, without being limited to the constraints of time.
Yes, there are rules on immortal magi, but the impact that they have on the ongoing story should be shaped to their now unchanging nature.
I am aware of the other methods of immortality and their comparative reliability, but I can imagine several narratives where a temporary postponement would be desireable, hence the question. Based on the response, evidently the Limit of Aging where magic "can slow it down and mitigate its effects" doesn't apply to Muto in any shape or capacity? Can it at least 'remove' the penalties from accrued aging points by transforming into a younger version of the person?
Then how does Muto work? It's a person's Essential Nature to not be made of rock, to eat and to breathe. Yet Muto can still turn them to stone for a month without them dying from deprivation.
Well, you are hitting two Hermetic limits, The Limit of Essential Nature and the Limit of Aging. Does it seem reasonable to you that a magus can simply turn himself into a fish or some other animal for a year to avoid the effects of his decrepit body passing on? Does he do this forever, and to what end? You can obviously do whatever you'd like, but you have to have an understanding of the endgame of such a scenario.
And they both explicitly mention exceptions; Muto must always be maintained for a change to Essential Nature, and magic can still mitigate the effects of aging.
That actually sounds like a fairly common narrative trope, especially since said magus's age and death will catch up with them the moment they're no longer a fish.
There are forms which can be assumed with Muto, for which concepts like 'dying' and 'Decrepitude' just do not make sense. Your troupe will have a lot of say on which these are - but a "thick, cohesive mist" or a part of a rock formation dying of old age are unlikely ideas. These are also forms which make any kind of activity as a magus quite impossible - so they may only buy a magus some time to wait for the Redcap with the vis to extend his longevity ritual.
Ok, so he's a fish. Now what? Let's walk through the implications of that and what is gained. Is he performing the ritual himself? Obviously, he doesn't care about the risks of botching, and does it without any mastery. What kind of animal does he become that still allows him to work magic and do lab work? Prior to the year being up, in preparation for a new ritual, how does he manage the ritual with a -15 to his casting total? How does he convince his covenant members to supply him the vis when he's not being a particularly productive member of the covenant?
What is the endgame and what is the story to be told by denying the inevitable. Mind you, I think there's great story to be told here, but ultimately, the mechanical aspects of this are the least interesting parts, and the endgame and what happens in between are really important.
And as much of a trope as it is to be sustained by magic, at death's door, it as much a trope to have that person so sustained turn to dust when the magic is gone. At a year after his natural death, it's perfectly appropriate for such a character to return to his natural state fully dead and rotting.
Yes, but I'm asking about the mechanical aspects to make sure I understand what's happening with the rules; so there isn't inconsistency between different storylines that use the same mechanical foundation. If I need help with a story or the endgame, I'll ask for advice when that bridge comes.