It sounds like your leaning towards...age at the rate of your essential nature?
otherwise...you get changed into a fruitfly with the life span of ~1-2 days. half a day passes, you are an old man? or would you be a fruitfly that lives for 50+ years? careful of yer answer fer the reverse also applies when you are changed into something with a longer lifespan.
I'd agree that the Limit of Time and the resultant aging is the critical issue here. Suggestions:
Make it a non-Hermetic ritual (ie like a Fenicil's Ritual from HoH:TL, Guernicus chapter) - being non-Hermetic it can break the Limit of Time, although I'd suggest the target gains a Warping Point per season for being under the effect of a powerful magical effect. Probably not a big problem for magi (until they wake up and start casting spells perhaps) but significant for mundanes who will acquire Flaws (and perhaps a single Virtue) from accumulated Warping.
Sleep in a time dilated regio (see Covenants in the section on Boons & Hooks) - if time passes slower, say 1 day for every 30 days spent in the mundane world, you'd only age 10 years if you slept beneath the earth for 300 years in a regio. Of course, knowing when to wake up is the trick - given most Hermetic spells have a maximum duration of Year, this may not be so problematic...
The precedent for this is in GotF with the Forest Path: The Path of Immortality (of course, you can't leave the forest regio in which you're in...)
Cheers,
Jarkman