I know that aging cannot reduce ability scores below their class minimums, but what of other sources of ability reduction? If, say, my bard is afflicted with the pallor of death as a result of mortality tampering, can her Charisma be reduced below 9? If so, what effect does this have on the character's ability to function as a bard?
Ability scores can absolutely be reduced below the prime requisite. There is no defined in-game result. You could have -5% XP penalty for 6-8, -10% for 4-7, and -15% for 3.
If you want to be more punitive:
A mage whose INT drops below 9 might be unable to learn new spells or read his own spellbook, since INT 9 is required for literacy.
A cleric whose WIS drops below 9 might be unable to recover spells from his god.