Magical aging

[Rubik]

If you consider third-party as a solution, then we can remove all weaknesses of everything. Just invent "Super Polymorph" as a 1st-level spell, make it instantaneous and pay a few bucks to have it put in a pamphlet with the d20 license stuck in the front cover and you're all set.

If it's endorsed by WotC, wouldn't it be 2nd party, at worst?

[Quertus]

Because there are no such spell.
Steal Life don't steal youth, it's just draining ability scores

You transmute the ability points you drain into youth. If you don't kill the person you're siphoning from, they don't get older, so it doesn't magically age them, but it is a very simple way to attain immortality if you've got 8ths and a reliable stable of people to drain. doing this and sponging off 4 points every month lets you maintain homeostasis and live forever, barring anything else.

Huh. The "subject withers and shrivels" just refers to ability drain, not to magical aging. Explains why no one mentioned it.

Although, as long as you are humanoid, there is no reason you can't target yourself with the spell, so you don't even need anyone else present to be immortal.

[Jergmo]

There is a third party book called Secret College of Necromancy that contains spells that reduce or increase a target's age; I played a 50-something Necromancer that used magic to maintain the physique of a 20 year old for the lack of Constitution penalties. I don't remember the specifics of it, though, as I lost the book. I think it's 1d10 years for either spell.

[Psyren]

If it's endorsed by WotC, wouldn't it be 2nd party, at worst?

If someone who was not WotC published it, I don't see a distinction that matters; either it's first-party or it isn't. And even if I did, this kind of "ex magic" is so nonsensical both fluff and balance-wise that I wouldn't take it seriously anyway.

[WhamBamSam]

Find a friendly dragon and cast it on them. Several times. Instant Great Wyrm!

Bestow Curse is Permanent duration, not Instantaneous, so multiple curses overlap rather than stack, I believe. You can absolutely get one age category that way though. I personally favor Mark of Justice as the spell delivering the curse, so the dragon can age itself as a free action in emergencies.

[Hobosub]

There is a Linnorm Death Curse in Pathfinder that hits at 1 year/day until cured.
Note that the 1 con damage/day is countered by natural healing