There are actually a fair few ways of longevity through the books already out...but you have to look hard(ish)
Lichdom: (Monster Manual): The classic. Great for the big scary evil boss.
Pros: Tons of bonuses, very few strings attached.
Cons: Messes up the social agenda a tad. Undead vulnerabilities.
Reincarnation: (Player's Handbook): The Reincarnation spell creates a new YOUNG ADULT body, so simply cast on the aged incumbent and make a new adult. I think this can't be cast on those who died of old age, so kill character first just before aging death.
Pros: Low-level spell, easy process.
Cons: -1 Level, Could come back as something undesirable.
Clones: (Player's Handbook): Clones created in your youth can be activated on your death, of old age or if n/a use tactic detailed above.
Pros: Assuming you maxout level 20 at young age, little negative impact. 100% guaranteed.
Cons: -1 Level, XP Cost.
Mind Switch (Psionic's Handbook): Find unlucky young victim. Switch minds. Kill victim. Keep body.
Pros: Quick, harmless (to you) and efficient. Could end up with better physical stats (switch with a top level barbarian?)
Cons: Negate Psionics.
Polymorph Other (cast by someone else on you) (Player's Handbook): Tell them to Polymorph you into a young X
Pros: Quick, cheap and (hopefully) reliable.
Cons: Average physical stats. Dispel Magic. Trusts person doing the Polymorphing (although if you have a handy cohort this is OK)
So there's five ways. There are probably more.