But no one has addressed the simple fact that a PeCo spell is not a "natural" state of affairs.
Feel free to ignore that detail if it makes a better saga for you and yours, and toss them on the same rubbish heap if you want, but the fact remains that PeCo and natural aging are two different things.
No, they're not. Perdo makes things worse, in this case by utilising natural capacities for getting worse. Creo makes things better, in the case of many healing spells, by doing the same. The flesh that regrows is not unnatural and cannot be dispelled. It is a momentary duration spell; the magic is over. The process by which the aging occured is unnatural, but the aging is not and cannot be so. Perdo can make a vase crack, but the fragments are not unnatural, even though the method used to break the vase was. Using PeVi on the vase does not make it whole again. As the change to Rego has demonstrated, changes in state/Essential Nature occur independently of the route taken.
PeCo can be used to open bleeding wounds. These would are not unnatural, and they are healed in the same way all other wounds are healed. PeCo can be used to strike a person blind by removing their ability to see; and this is an unnatural destruction which persists for the duration of the spell.
You can argue that Hermetic magic should be able to repair aging - there isn't actually any reason against is save a meta-setting excuse to get rid of immortal magi. In this case, you would be able to use CrCo to become younger again. You would not be dispelling the aging spell, however. To be honest, there isn't any real need for the limit and you can make Longevity rituals a bit more powerful without major impact; Warping will still be the real limit, combined with the Order's noninterventionist policies. The only real issue is that decripit magi tend to provoke better stories than an order of happy, healthy youths.
Sez you.
It seems that this may be one of the rare few times that I agree with CH. If the aging was caused by magic, then it should be able to be reversed by magic. Seems fair to me.
Decapitation can be trivially caused by magic. Undoing it is beyond the scope of Hermetic magic.
The issue in this case is that this is a case of Essential Nature. The victim is now older, and cannot become younger by Hermetic Magic. You could use Muto Magic to suppress the effects of aging, but not Creo. To raise a tangential point, you can age people with Creo, but only until their late teens. During childhood, aging is maturation, rather then decay, and so a baby is immune to PeCo aging spells, though not to most others. Would you allow a baby, aged to maturity, to be returned to his original age? Hermetic magic, by the RAW, cannot do this. It breaks the limits of aging and essential nature.
You can artificially age someone with Muto Corpus, and these effects can be dispelled, but they are also effects with duration. PeCo aging merely kicks a natural process into overdrive and laughs.
OK, I guess I'm just in the minority. It doesn't matter for me that the aging 'came' from PeCo. Once the spell is over it is no longer an active magic and what is left is natural. Making a distinction holds implications for the nature of momentary Perdo effects. Natural is what things are after momentary Perdo effects happen. When the caveat of 'natural' is put in limit description, I read that to mean things not under active effects like curses etc.
The thing is that being old is not part of the essential nature of the aged person IMO. It has been aged, yes, but he is still 25 years old, even if his body appears to be 80. So, it can be reversed WITH hermetic magic. The base for this spell can make it prohibitive, though.
Otherwise there is no point in diferentiating if the aging was natural or not in the guidelines.
Cheers,
Xavi
<nods, crosspost>
People are confusing something that is "natural" with something that is of one's "Essential Nature".
I return to my analogy of the loss of a limb vs. being born without one - the end result appears the same, and may have all the same penalties, but the one is "Essential Nature" and cannot be treated with magic, and the other, altho' certainly "natural", is not and can be easily repaired with magic.
It's not magic vs. natural that's at issue here - it's whether the aging in question is part of one's "Essential Nature". And has been said (many times!) before, there's nothing of "Essential Nature" in a PeCo spell. If there were a mundane way to age someone - being badly scared, or being struck by lightning, perhaps - I'd argue that those, too, are aging that is outside of one's Essential Nature, and could be reversed.
"Natural" aging requires time - that's the only "natural" aging there is, that is part of Essential Nature. And since the limit applies specifically only to "natural" aging, it's logical to deduce that 1) there is unnatural aging, and 2) unnatural aging does not fall under that limit, this latter being further supported by the connection of the Limit of Aging to Essential Nature - which cannot be changed (or brought about) by any spell, the "instant" nature of Perdo notwithstanding. 
Hi,
Death too!
And burning down a forest through CrIg should also be reversible by magic, to return the forest exactly the way it was.
And if a pit is gouged in the earth using PeTe, the lost terrain should be recoverable through magic too.
Not.
Also, aging a 45yo man by 5 years instantly should immediately inflict 5 Aging Rolls. That's what aging is in Ars Magica, not simply erasing the 45 on someone's character's sheet and writing in a 50.
But YMVs.
Anyway,
Ken
It can be. Iirc, it's called CrHe and CrTe.
Just as if a person's arm is cut off, that arm can be replaced with CrCo.
But no spell can change a person's essential nature if they are born without a limb. And no spell can change a person's essential nature as to their "natural age".
So unless the PeCo "aging" spell is somehow changing a person's Essential Nature, then a CrCo spell can either reverse it or replace it.
That's how it works.
Hi,
Except that Hermetic Magic explicitly cannot remove Decrepitude. Except that it is explicitly not known that the Limit of Aging is due to the Limit of Essential Nature. Except that the only CrCo guideline that exists for aging allows only the removal of the effects of aging, not the aging itself.
Anyway,
Ken
I think, rather, that we differ in what we consider essential nature.
I disagree. The ruling you refer to is entirely a meta-game construction to stop people getting rid of cheap flaws. Certainly once a wound has healed then I would rule that it is, because the person's body has accepted it, part of their essential nature. A man who lost a limb as a boy cannot be healed as easily as a grog whose bandages are still red. There are mundane ways to age people - time does the job nicely, and its effects cannot be reversed. The Essential Nature in question is not the age itself, but the capacity to age and the incapacity to become younger. My essential nature is that I am human and cannot spontaneously grow wings of fire, but will grow old and die; it is not that at this moment in time I am in my late twenties and sitting at a computer desk. The latter are transient aspects of my nature, but not fundamental to it; there's nothing essential about them.
Both our stances are based on definitions of age. If you define age as a chronological term then yes, the aging is unnatural. If you define age as a stage which is normally naturally reached via time then it is not. I would argue that the latter is the case for Mythic Europe. If this were not the case, then virtues which slow down the aging process and immortal, unaging creatures would not make sense. These beings are immune (or highly resistant) to aging spells and it has nothing to do with their elapsed lifespans but because they don't age in the normal, human sense. Were aging (in the physical sense) inherently tied to chronology, an Hermetic magus would not have different apparent and actual ages. If you hold the aging to be unnatural then of necessity a Perdo Vim spell at a magus will strip his Longevity Ritual and he will return to the physical age he should be and likely die horribly. The magic affects the process, not the state. Since Creo cannot age a man past the age of 16ish, and Perdo cannot age a man before that, the aging is a normal part of the Essential Nature.
Water frozen by a ReAq spell is frozen water; water frozen with MuAq is an unnatural substance. Water frozen with PeIg is just frozen water - there is nothing unnatural about the cold, just the method of cooling. An icicle frozen by a momentary PeIg spell and thrown by mundane hands will pass neatly though the Parma and spear a magus.
And then this (and the pearl thread) is when I flee the boards and sob in a corner for not having a PhD in Medieval Science.
Ain't it great when we all confirm the bad reputation of Ars Magica by our actions?
Xavi
Hi,
easy laughter We create the bad reputation of AM by our actions. We are the Cause of D&D4....
Anyway,
Ken
p. 129, Restoration of the Defiled Body
I most certainly hope not! D&D 4 is the crappiest RPG i´ve seen for a loooooong time. D&D 3 had lots of good sides going for it but .4 just sucks.
. 129, Restoration of the Defiled Body
yeah....I've seen ppl post in this thread that you can't reverse aging no matter what...BUT yet this spell clearly says it reverses premature aging. Anyone that can't explain themselves about this spell needs to back down. If you can Ill take a pen to my book and scribble it out. OR you can conviently ignore the posts calling attention to the spell in the rules book.
Hi,
It removes the effects of premature aging. So you get back your lost characteristic. But not Decrepitude. And if an aging spell kills you, this spell doesn't bring you back.
Anyway,
Ken
OH! Redefining Essential Nature! Sneak Attack, +3 for Surprise! 
Well, in the book, there is very little that does "define" EN - most seems to be what "is", rather than what "can be", but that's not to say both aren't the case.
Hrmmm... well, I think you've planted a flag where it is unassailable, at least from any canon material, even if it is also unsupported by same. Fair enough. 
Oh, I disagree - it's the need for personal consideration of such subtleties that is AM's strongest point! Most players of that-game-that-shall-not-be-mentioned don't think twice about the how's and why's, and so when two similar oddities show up they're likely to rule opposite half the time, a literal coin toss. But in Ars, there is a constant "logic", and where in that other game literally anything can happen and for any reason* and simply be accepted as a typical day, in Ars there are predictable cause-and-effect explanations and limitations.
(* A portal appears in your bedroom one night, and an entire phalanx of Amazon cavalry pours through riding mechanical dinosaurs on their way to some unrelated conquest, excusing themselves politely and leaving a diamond the size of a pumpkin for your trouble. And so you go back to sleep - after all, that's the third time this week... 
)
Heh, yep. When in doubt, rtfm. (Who said that?)