Additional spell guideline as in extra guidelines that would exist in 1220, or extra guidelines that someone might invent with a breakthrough?
I don't think I have any of the former, but I do have one of the latter. It's essentially the proper integration of the same magic used in longevity rituals:
Creo Corpus 5: Grant an ageing bonus of +1. The spell must last for the majority of the year for this to have any effect.
Creo Corpus 10: Grant an ageing bonus of +2. The spell must last for the majority of the year for this to have any effect.
Creo Corpus 15: Grant an ageing bonus of +3. The spell must last for the majority of the year for this to have any effect.
And so on.
It was the output of some original research in a saga. The logic behind the balance is that it should end up being weaker and more situational than longevity rituals at the higher end, but more versatile and cheaper at the lower end.
- For small bonuses and if enchanted into a worn item with a constant effect it causes 1 warping per year, essentially working out as a cheap and re-usable longevity ritual (because it doesn't need remade if a roll is failed). The maximum bonus you can get with this before it becomes a powerful mystical effect is +3 to aging rolls.
- For bonuses of +4 and above that approach becomes cheaper in vis than a standard longevity ritual, but it's impossible to achieve an equivalent bonus because you have to waste levels on duration, making the effect constant etc. You also can't spend extra vis to boost your lab total. As it's enchanted into an item it can be stolen or destroyed in a way that a longevity ritual can't. Unlike the item described above it's only good if designed specifically for one person - otherwise as a constant strong effect it will inflict 5 warping per year!
- Casting it as a ritual spell is totally impractical because you'll end up spending far more vis on yearly rituals than a single longevity ritual would take. However, this might change if combined with year+ length durations like "Until".
It ends up, in theory, complimenting rather than supplanting longevity rituals. You still need a proper ritual to get the best protection and that's what older magi will opt for. But for younger magi or those very concerned about cost or their fertility enchanted "rings of longevity" or the like tend to be used. Some older magi might have an enchanted item designed as a fall-back for if and when their longevity ritual fails while they arrange for a new one to be cast.
IMS house Mercere are the biggest fans, because they can make/buy a bunch of "give +1/2/3 to ageing" items that can be passed down through families and don't impede fertility.