The scaling when adding mags is difficult to deduce! aside - maybe look at the CrCo guidelines and look to the types of aging crisis resolved too? PeCo should probably be easier/better than that relative Creo power.
I'd guess that killing somebody in terms of aging is an equivalent result that is likely to render them more far more long term harm than just an incap wound.
Even young adult is going to be ruined by aging when hit with a spell inflicting a 50+ year advance in a single moment. That's devastating. Aging the target to 135+ is massive/epic.
Perhaps 15:5 years, 20:15, 25:30, level 30:50 years, 35:75 years...depends on how scaling should work according to the troupe.
It could also be 15:5, then 20:25, 25:125, etc too. I can see a reason why either is ok.
Consider too the effect on character who have longevity rituals, where 30 years is a major pain but might not kill them. How awesome should the spell be once it has penetrated against that foe?
Perhaps too we might also consider that just because the PeCo guideline says level 30 is killing somebody, that's (probably) to indicate the general severity and difficulty of what magic can do with direct application of that level of power. An equivalent lower level guideline doesn't have to be death just because it's extended to that level.
That's a tough question. I can see level 30 as aging someone 120 years, based on the "maximum human lifespan = 120 years" for Fallen man and that being equivalent to killing him. But I actually think aging someone to death should be a bit harder than just killing him. So I'd suggest putting it at base 35 or 40.
So, I'd suggest the guidelines:
15 - age someone 5 years
20 - age someone 20 years.
30 - age someone 50 years.
40 - age someone 120 years.
Base 30 should still be enough to kill lots of people. But it takes base 40 to really do it for sure. And base 20 is serious, but not TOO much greater than base 15 and leaves most people still functional.
I would only make a couple of additions to the aging guidelines for spells.
"Threescore and ten" years seems like a good/appropriate one to add to mainstream Hermetic magic. I'd probably make it a level 30 base, the same as killing a person.
I'd also consider a guideline or three for Faerie Magic that ages a target to the cusp points mentioned in the Becoming Mystery under "Transforming the Body" (HoH:MC, page 93), if none exist. The base level for that could be a little more complicated, though.
I would strongly recommend just using the existing "kill someone" for level 40, with "aging" as the means by which this happens. On the one hand, this would not kill someone immune to aging, such as various immortal people. On the other hand, it lets you benefit from a Hermetic Focus with aging, and lets you kill Nephilim and other long-lived people. (Sort of the way clenching someone's heart doesn't help if they somehow don't have a heart, or if they have a heart of stone (nyah, nyah, you need Terram.))
On the surface, a guideline @40 to age someone 120 years seems reasonable. But do you really want to stop the game to roll 120 aging rolls?