Someone with a longevity potion is under a constant magical effect, by definition. How is that not equivalent to a long-term MuCo shape-changing spell according to the RAW? (I don't like it, and I HR it, too, but if it's incorrect according to the RAW, that is even better.)
Longevity is just different.
However, there is no exception for it in the RAW, as far as I am aware.
The RAW as written even explicitly clarify that the Longevity Ritual is an on-going magic effect, which is why you gain warping from it. So, according to the RAW a character who has taken a Longevity Ritual should be resisted by Magic Resistance.
This is particularly irritating for Bjornaer, as it means that the assorted claws and teeth attacks of their heartbeast form are quite useless against targets with Magic Resistance, once they have taken their Longevity Ritual.
One could always say that the Target of the LR is your Life Force, not your physical body. Nothing physical about you is being affected by magic, thus no need to worry about Magic Resistance.
You could if you want to.
But that seems a bit unlikely as the actual effect of the Longevity Ritual is to prevent/slow the physical aging of your body. And LR is a Corpus effect, which is (usually) about physical bodies.
Usually, but Life Force is mentioned twice; "Transfer Bodily Energy" (ReCo), and the fact that you are rendered infertal because your life force is bound up in longevity (don't recall the exact wording).
Simple answer though. Everyone who plays Ars Magica, including the line authors, don't subject LR's to Magic Resistance. It may be an error of omission, but it simply just works that way.
That still doesn't get you around the fact that the primary effect of the longevity ritual is to slow/prevent the physical aging process. It's a physical effect.
Anyway, even if it targeted your "life force" (whatever that is), you would still be subject of a magical effect, which is the important thing for whether you are resisted by magic resistance.
Well, you do have a point there. I would say that, rather than try to dance around a bunch of fictional magical metephysics, it is easier to just simply say "Longevity Works Different, and it was an oversight to neglect mentioning it".
The first two only cause warping if you remain in animal form for a year (it's not a Powerful Mystical Effect, but it could be a Continuous Mystical Effect). Shapeshifter does not cause warping for either - the magic takes place at the point of change, like the Heartbeast.
Mark
Aren't all those high magnitude spells/effects, and so cause warping? No for shapeshifter, since it is clearly defined for your own use, but the other ones?
Xavi
No - Shape of the Woodland Prowler is MuCo 25. With continuous use in a device (which is what Skinchanger effectively is) is MuCo 29. Just scraps through!
Mark
A Bjornaer is in essential form, so there is no Parma that would protect from its claws.
Xavier
Please read the whole context of what you have quoted.
Yes, normally you cannot resist the claws of a Bjornaer in heartbeast. However, if the Bjornaer happens to be the subject of an on-going magic effect (not the heartbeast, something else) then his attacks do need to penetrate magic resistance to be effective. This would be the case, for example, if he had a spell cast on him to make his claws sharper, or his hide tougher, or his fur pinker.
If you play that the Longevity Ritual is an on-going magic effect, which is what the RAW explicitly state, then once the Bjornaer has quaffed his Longevity Ritual his claw attacks are resistable.
(Arguably.) 
It was unforeseen. Deal with it as you wish. Personally, I've house-ruled it away long ago.
Does it make a difference if the source of Might is from a non-magical realm, such as Infernal Might?
The effect of shapeshifter, skinchanger, Lycanthrope, MuCo spell, and magic item are actually address in the HOH:MC p22.
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those changed by spell (MuCo) or invested item (including skinchanger virtue) are under a spell and must penetrate magic resistance with penetration of the effect to touch or attack any person.
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Those changing inherently (shapechanger, lycanthrope, etc) are only under magic at the time of the change. Thus they do not radiate magic, suffer warping for the time spent in animal form and do not have to penetrate MR for mundane attacks. They are subject to Me and Co effects as well as An effects.
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Heartbeasts: This is same as category 2 except that they are not subject to Co or Me effects since they have truly become animal.
Thus shapeshfiters, lycanthropes, heartbeasts and other innate magical transformations don't face magic resistance when attacking.
The concept of the longevity ritual stopping physical contact is silly and I hope that there is errata added to insure to make sure this is not the case.
I agree about Longevity, it shouldn't need to penetrate.
But imagine, if two magi wanted to be physically intimate, they would have to let down the parma first. How poetic 
At one point in history of the rules, Parma also screened out ~any~ subtlety of detail and intensity of the world, so any exceptionally bright colors, fine flavours, artistic impression of music, human touch, etc etc were muted, encouraging them to drop their Parma any time they wished a "fully human experience" of the world. Something about the Forms "protecting" a mage from everything and everything. In many ways I'm sorry this did not carry thru to the new edition. (I'm sure it was great for a hangover.)
I have a question on this topic I've always been curious about and would like to get some opinions please.
In the fifth edition write up for the spell Edge of the Razor, there's a comment that any sword enchanted by the spell would be resisted by MR.
I can understand a magi's parma negating the razor portion of the sword. But it resisting the sword itself (assuming the sword was the result of craftsmanship, not a CrTe spell, etc) I must admit puzzles me. Even without the edge, a sword is several pounds of iron being swung with some force against flesh and blood.
If I'm reading the string of posts correctly, it seems the consensus on the issues relating to the dragon and shapeshifted characters is that parma stops magic but the flesh and blood portion (the dragon's claw, the shapeshifted wolf's fangs, etc) will get through. Wouldn't that same logic apply to inanimate objects as well?
I've played Ars but not to a real depth and this issue has always confused me, particularly since it's handled in so many different ways. If anyone can help set me straight, it would be appreciated.