In DnD 3.5e, is there any way that I can take an "venerable" character, which takes a -6 to all physical stats and +3 to all mental stats, and UNDO the physical stat losses? This character is a level 9 wizard.
The Polymorph spell will give you a shiny new set of physical stats (but will not change your hp - that is what False Life is for). Presumably also you will be a Dragon or a Troll or something fancy.
The Reincarnation spell makes you young again, but presumably, removes the aging bonuses (nothing about this is explicit, but uh, hrm).
Be a Dragonwrought Kobold (and very, very old) for mental bonuses with no physical penalties.
Use Astral Seed and shenanigans to have your body 'reform' as that of, for example, a Great Wyrm Gold Dragon.
Have a higher leveled caster cast Polymorph Any Object on you to change you into a younger version of yourself/humanoid race of your choice. Duration is permanent, so although it can be dispelled, you won't be old even when you're asleep.
Become a Necropolitan, and never worry about that pesky Con score ever again.
Wait a bit, and then, become a Lich. Ergo, etc.
I can't recall anything that explicitly says that intelligent undead don't suffer physical aging penalties but benefit from mental aging gains, but it seems pretty reasonable.
Even getting bit by a vampire or lycanthrope is not a terrible idea if you don't plan to gain levels much (short duration campaign).
If you have Persist Cheese active, persisting a single buff like Bite of the Weretiger will trivially solve a lot of those issues by hugely boosting your physical stats and toughness.
If you want my advice, though, roll with having poor physical stats and make party members carry you around and stuff. Sounds hilarious and fun as hell to roleplay.
Last Breath, Spell Compendium
School: Transmutation Class/Level: Druid/4 Components: V, S Casting Time: 1 standard action Range: Touch Target: Dead creature touched Duration: Instantaneous Saving Throw: None Spell Resistance: Yes (harmless)
Description: Last breath restores life to a recently deceased creature, creating a new body for the returning spirit to inhabit. However, the spell must be cast within 1 round of the victim's death. This spell functions like reincarnate (PH 270), except that the reincarnated creature receives no level loss, no Constitution loss, and no loss of spells. The creature has -1 hit points (but is stable).
Material Components: Rare oils and unguents worth a total of at least 500 gp, spread over the remains.
You can use Last Breath to reincarnate without CON or HD loss. Unlike Reincarnate, you can spam Last Breath indefinitely.
If you have time, you could also multiclass into Green Star Adept (Complete Arcane) and become a construct after 10 levels, or into Monk or Druid and lose all age penalties after 17 or 15 levels, respectively.