So I have a concept that fits a couple of genres.
All of them use the virtues death prophecy and unaging.
Could be a holy fighter that was blessed on crusade or a solider that made a deal with demons or a warrior that was gifted by powerful fey.
My question is, how old can you make a character? Does this scenario work? A Nigh unkillable companion, is there anything I’m missing?
You still make the aging rolls, you just don't get the bad effects, but you do die when you get that really bad aging roll. You could make your character quite old, say 50, but he is definitely closer to death by aging than other characters, so it does not really meat the Highlander design.
Character hundreds of years old are unbalanced because their XP will be ridiculous.
I don't object to the idea of a functionally immortal companion, if that's their only supernatural feature and the design is decently constrained. The whole of their virtues would have to be poured into the central concept, and with flaws to match.
One issue is that some abilities become obsolete. You are also stuck moving along every once in a while, in traditional fashion.
Still, you'd be very good at a few things, even with the extremely high cost of improving an ability after a few steps on the point pyramid.
Mechanically you can have a spirit bound to you by using Infernal Binding. In that case you accumulate Warping - 4/ Year - but that doesn't trigger Twilight. You use the spirit's power to keep your vitality, therefore doesn't roll for Aging 
I can imagine that the Divine can give you similar blessing's without the bar effects of Internal Warping. But you have to deal with Divine Warping.
Or you can go with Strong Faerie Blood and tinker it a little bit
Its intentionally designed I think that most characters are not immune to death.
The closest way I can think of to make a character live forever is bind a spirit to them or get immortality of the forest, then a lesser immunity (warping from life-extending effects).
Mind you, taking immunity to warping in itself is like taking immunity to damage- so rather than that, it is more appropiate to take an immunity to sources of warping and to note it specifically applies to warping.
If you don't mind playing a baddie, more Kurgin than Highlander, there's a way in ROP:I. Take Incantation+Consumption, along with Unaging. Then you can use I+C to transfer away decrepitude and aging crises by being a real bastard.
Death prophecy stops you from dying until its condition is met. Old age will not kill you.
Is there something in there this combo misses?
I thought unaging means you don’t take the negatives from aging rolls? Decrepitude builds but cannons kill you due to the death prophecy.
I suppose the only limit to your age(experience points and abillities) would be what the story guide and troupe decide.
You are right, and I am wrong. They would work.
Starting with a 500 year old character, and over 7500 XP would be a tad unbalanced though. If the group is happy for the character to exist, I believe the rules would allow it.
I was not sure if I had missed something, thanks for confirming. I think there would definitely need to be some limits on age and potentially reduce the exp per year(take some flaws that make it harder to learn since you might lack motivation). think the first run at this character I had rapid convalescence to help with healing.
The scenario certainly can work.
The idea of a character being "unkillable except from a specific source of danger" is specifically what's behind the Death Prophecy Virtue. However, the authors did recognize that, in its generality, Death Prophecy can be abused, so it's one of the very very few V&Fs that explicitly requires troupe/storyguide moderation: "Players may only take this Virtue with the agreement of the storyguide or troupe". Very few troupes/storyguides would agree to a 2000-year old Companion with some 2000 x 15 = 30000xp!
I think the only problematic issue here would be if a character were created with an extremely advanced age and the corresponding amount of xp. For such a character, I would just assign a flat number of xp, as per Magic Characters in RoP:M - with the absolute maximum being the 1200 xp of a Winter character. This can be explained as the character being actually relatively young, or having forgotten much, or having a lot of outdated and now effectively useless knowledge. In fact, I'd point out that such "Immortal" characters are best designed directly as Magic Characters, or perhaps Faeries.
That is a great idea to limit the exp. I agree the linear 15 points a year breaks when a player places all of the points in 2 skills.
I think my troupe is specifically wary of magic characters and faeries.
One of the players created his animal companion for his companion. It worked out being a companion level character with ten points of might and all of the animal stats for free.
Our story guide is creating our familiars and not using any of those rules.
Every Death Prophecy comes with a loophole that can kill you. If someone wanted to play this concept, I'd make them roll each past year for a chance of the Prophecy having been fulfilled. (Maybe a 1% chance each year?). If that happens, oops, goodbye.
And when you got to Decrepitude 5, the Prophecy would be fulfilled somehow. The Angel of Death would make sure of it.
From a narrative perspective, one of the things about Death Prophecy is that You Will Die Of It. Don't try to cheat divine justice.
That is not a bad idea about limiting it, my story guide said the same about decrepitude at first. I think your concept negates the virtues to a big degree. Might be better to just disallow the character concept.
The problem with "outdated and useless knowledge" is that ArM Abilities tend to be broad, and tech hasn't been advancing all that quickly. Yes, sure, Area Lores and Organization Lores do die, but . . .
Consider someone 900 years old, who, because of lack of motivation to actively learn anything, has only been gaining 2 Exposure XP per season (8 XP/year). Now, we further assume half of that somehow wound up in various now-useless Abilities, or replacing things that went useless with time (like his original Native Language). That still leaves 900 years times 4 XP, or 3,600 XP total.
Let's set aside the two-skill concentration idea, since this guy lacked the drive to do that. Instead, he just did a whole bunch of different things as he lived his long life, and he wound up with ten 8s (Animal Handling, Awareness, Carouse, Charm, Folk Ken, Guile, Hunt, Ride, Single Weapon, Survival), ten 6s (Bargain, Brawl, Craft: Carpenter, Craft: Farmer, Craft: Stonemason, Etiquette, Intrigue, Leadership, Profession Merchant, Stealth), six 5s (Area Lore: Current Location Now, Athletics, Craft: Blacksmith, Great Weapon, Living Language A, Teaching), and six 4s (Bows, Chirurgy, Concentration, Living Language B, Swim, Thrown Weapon.)
He's not utterly supernatural in anything, no, but, he's incredibly broadly competent at all sorts of practical life skills.
To bring such ancients down to Earth, a realistic "Abilities not in regular practice decay with time" rule would work; the shortcut to do that is to fiat-limit the immortal to some reasonable number of total XP, on the theory that he's losing as fast as he gains.
IIRC in the RoP magic they talk about how magic might beings progress through transcendence? I think that a system like that could be used.
I think the character is much cooler at an older age but, I also don’t think they should benefit fully from the exp.
I’d even make a flaw(only gains exposure exp)
What amuses me is that "when your neck meets a blade, on that day you will die" is a very weak usage of death prophecy, and would have to be bolstered with other virtues to keep them from suffering decrepitude and aging to where killing them would become trivial.
On the other hand having "immortal" faeries that take over a person's life when they are first killed and run around acting like highlander immortals seems to be a pretty good fit to the game (when "killed" they find someone else's life to take over when they die)
I love the idea of a fearie band. I had the concept of Arthurian fearies or Robin Hood.
How do you make immortal fey? I think a magical or fey creature would be a good way to get some of the powers from Highlander.
While an Immortal like this should probably be a Magic creature (eg, the Living Goddess from RM) I like the idea of it being a Virtue with a mandatory drawback.
Loss of abilities due to forgetting as you age-without-aging seems like a good way to go about things. You could essentially modify the normal aging table so that on a roll of 3+ you take a hit (let's say 1 Ability Score Loss of XP in that Ability) to any one Ability you didn't use that year, or didn't substantially use, with a Crisis being the same but for several Abilities. I don't want to deep dive into the maths for that table to keep things relatively balanced, although I think it works for a character who would be an absolute beast with a very few specific Abilities. Because of the "+Age/10" factor, at a certain point you would almost inevitably be having a Crisis every year, meaning that mathematically there would be a Hard Cap for how powerful the character could be. They'd still be able to be an absolute beast with whatever their Main Thing was.
Normally when aging you subtract The Longevity Ritual Modifier, perhaps this is substituted for a subtraction of some value from the last Immortal you killed (I would suggest the Might, although I don't have a good substitute for non-Might Immortals with the virtue. Maybe their Age/10 for symmetry.)
I do think no matter what there would be some people out there who would be aware of these immortals and find them an affront to (God, Natural Order, Hermetic Arrogance, State Power, or some combination of the above) who would make it their personal mission to seek these people out and kill them with hammers as quickly as possible, and unlike the normal Highlander Universe there would be a lot of people capable of doing so.