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.
[RPG] How to undo the physical effects of aging while keeping the mental effects
With an ultimate magus, it’s more important to keep yourself as close as possible to a single-classed spellcaster on one side than it is to keep the two even.
Thus you want to circumvent as much as possible ultimate magus’s attempts to progress your lower-level class.
Because at 1st, 4th, and 7th level, ultimate magus advances the class with lower caster level (your choice in the case of a tie), this has a tendency to even out your two classes. If you avoid that, and continue to progress in the more advanced class, you will have more of your highest-level spell slots, which are also higher level than they would be.
Practiced Spellcaster is the solution to taking manual control over which class ultimate magus progresses. Practiced Spellcaster increases a class’s caster level (but no other facet of its spellcasting) by up to 4 (capped at the spellcaster’s HD, so the feat is only beneficial to multiclassed spellcasters). Normally, caster level is nice but not as crucial as things like spell slots or higher spell levels. Ultimate magus switches this up, though.
If a Wizard 4/Sorcerer 1 takes Practiced Spellcaster for sorcerer, his weaker class, his sorcerer caster level is 5th. This puts it higher than his wizard spellcasting’s 4th caster level, so first level of ultimate magus advances wizard. This is despite the fact that the ultimate magus’s wizard spellcasting is, for most purposes, more advanced than his sorcerer spellcasting.
Now you have some control over your advancement, and that allows you to maximize your spell slots.
At 4th level, both classes are even, so you should choose wizard. At 7th, barring finding some other bonus to sorcerer caster level, though, you have to accept some sorcerer progression, which is a shame but not the end of the world. You’ll end up “missing” a total of two wizard spellcasting levels (compared to a single-classed wizard), but gain a total of nine sorcerer levels, plus of course all the rest of the ultimate magus class features.
Otherwise, you need to improve the abilities that grant you bonus spells. Consider the beguiler (Player's Handbook II) instead of sorcerer: you'll use Intelligence for both classes.
Versatile Spellcaster is another feat that will make your slots more flexible.
And pearls of power and rings of wizardry can help by letting you reuse slots. Memento magicka are like pearls of power for spontaneous spellcasters but they are 50% more expensive, which is badly overpriced.
You can remove the multiclass XP penalty by playing a race with one of those as your favored class (like Elf), or as Human, where your class with the most levels just doesn't count no matter what it is. Human is pretty much always a good choice anyway.
And that's if your DM actually uses those rules. In my experience, quite a lot don't enforce those XP penalty rules.
Class Choice - Look at Beguiler
Sorcerer would work as a first class choice. Beguiler (Players Handbook II) would work even better, as it's also a spontaneous caster, only one whose key stat is Intelligence (same as the Wizard). This plays off a stat you already need and fits the idea of a "learned" character who also has innate ability.
Add Ultimate Magus as a Prestige Class
If you go Beguiler/Wizard or Sorceror/Wizard, you can then go into the Ultimate Magus (Complete Mage) prestige class, and boost spellcasting from both at the same time. One would go up faster than the other, but the class also lets you use spells from one to power metamagic from the other, and it can do some pretty neat things. See also What exactly are Prestige classes, and what purpose do they serve?
Instead of just taking the spontaneous spells as a one off thing, this lets you develop both of them as the character learns to blend his two types of magic together. It's a pretty neat (and powerful) concept.
This answer has some useful information on how to get the most out of Ultimate Magus.
Wizards can get the ability to cast any Divination spell spontaneously by trading their 5th level bonus feat for this ACF. It does what it says - you can trade any prepared spell you have to spontaneously cast any Divination spell you can cast of equal or lower level.
Other Option - Reserve Feats
If you don't want to multiclass but still want some spontaneous abilities, reserve feats are another option. These let you use some power so long as you have a spell "in reserve" that you don't cast. Examples are things like being able to do small flame strikes, shoot lightning, summon elementals, create force darts, or do some minor magic disruption. You can get that as early as level 3, and you can use them infinitely so long as you don't cast the reserved spell.
It's not as versatile as true spontaneous casting, but it also doesn't require multiclassing at all. So I felt it was wroth mentioning.
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.