I would suggest that the 50th level effect be similar to the Mercurian Ritual Purification, found on page 97 of Mythic Locations:
Purification
CrCo 40
R: Touch, D: Momentary, T: Individual, ritual
This ritual may only be cast on another character who has a Decrepitude Score of 1 or higher. The caster cannot cast it on himself. During the ritual the target must be bathed in water, in a consecrated temple of Mercury.
The target’s apparent age is reduced by 5 years and his Decrepitude Score is reduced by 1. The player also removes up to 5 Aging Points currently allocated to the character’s characteristics.
Assuming that the vis is available, and the caster is willing, the ritual may be cast multiple times in quick succession (to reduce a high Decrepitude Score to a much lower value). The ritual has no effect if the target character has a Decrepitude Score of 0. Note that the ritual does not reduce the actual age of the character, so it has no effect on the Aging Roll.
Commentary included in the Lab Text suggests that the Mercurian priests used this ritual in a similar manner to the way Hermetic magi use the Longevity Ritual. The ritual is compatible with the Longevity Ritual.
This ritual breaks the Hermetic Limit of Aging.
(Base Effect, non-Hermetic)
The 50th level version created from studying the Fountain would remove 3 points of Decrepitude (+2 magnitudes over the 40th level version that only removes 1 point), but could only cast in a structure consecrated to God. (The Dominion aura will be a problem for casting.) It's a powerful effect, but I think that ultimately removing Decrepitude is less valuable than removing Warping. Conquering both would allow for mages that could live forever without changing into an immortal form with Might.