Yeah you could have lot of fun with the idea. An initiation that involves a high powered fairy whisking a character off to Arcadia glamoring them into a baby and reliving early childhood under the care of faeries would be an interesting adventure.
It should also be noted that the Magical Realm version of Faerie Blood, Magical Blood, can come from mere exposure to the "Magical Realm" and not just birth. So initiation should usually be an option there. Alas it lacks a Strong Magical Blood Major Virtue version though so no forestalling rolls for 15 years. But then their is the Transformed (Being) Major Virtue. That forstalls aging rolls forever. And a canonical initiation exists. But good luck getting it.
One thing on Longevity Potions; they render you 'permanently sterile'. I've seen no rules that suggest 'permanent' = 'irrevocably', any more than someone 'permanently' blinded by gouging could not have their eyes restored by magic.
A ritual CrCo spell could remedy the sterility, though probably not as long as the potion is in effect. The idea is supported by Art and Academe; hermetic magic is capable of forming both cambion and sperma (pg36, "Babies of Hermes" inset). While Hermetic magic can't create embryos, restoring reproductive health should be well within Hermetic Limits.
Other longevity substitute: the Divine power Blessing. Consult with your local (?) Holy Magus.
Apart from the dictionary definition of "permanently", of course.
But sure, magic of some sort might be able to reverse the damage. However, the fact that magi seem to treat it as permanent, implies it is not simple to undo via vanilla Hermetic magic. You are probably running into the Limit of the Soul.
Storm Wizards (Translyvania hedge tradition) have a Mystery initiation to gain Giant's Blood --- initiation involves hunting, slaying, and eating the heart of a giant (Against the Dark, page 127).
This in a game where the previous edition had Permanent duration spells - that could be dispelled? Permanent in a RPG just means until some OTHER wizard gets involved.
As I wrote, you'd need to let the longevity potion lapse (probably), which magi are understandably reluctant to do (and then there's a ritual spell involved too). Note there are canonical examples of people in Ars having children despite the Longevity Ritual (the bad folks at Dankmar), though they're using a faerie enchantment. So yeah, 'permanent' isn't permanent. Does the LR alter the Essential Nature of the magus? If so, why aren't magi investigating the LR as a source of insight into breaking the Limit of Essential Nature?
I once had an NPC magus have his LR lapse, and then only became fertile after he went without it for an equal length of time as the LR had been in effect.
