What does a Character lose when his mental stats go below 3?

Question

Is there a minimum intelligence requirement for taking class levels?

If so, would a character reduced to very low int also temporarily lose his class levels?

What are the requirements for feats as well?

Would a PC be turned into a NPC?

Answer 1

The simple answer is the rules aren't terribly specific on this. It's mostly a question of proper roleplaying and common sense, one of the things the rules don't really have to cover explicitly. The only thing the rules really strip you of are feats (if your new Int means you no longer meet the prerequisites) and languages (Int 1-2 characters can't use language at all and Int 3 characters only understand it but can't use it, from the Attributes section of the core book). Even if they somehow acquire languages (i.e. from Linguistics skill points) they can't actually use them unless they have an Intelligence of at least 3 (for understanding) or 4 (for speaking or writing).

Okay, let's try and run through these...

Is there a minimum intelligence requirement for taking class levels?

RaW, no. However, if you're roleplaying even vaguely well, a being with extremely low intelligence is unlikely to gain a class level in (almost) any class. That generally requires focused study, training and intent.

If so, would a character reduced to very low int also temporarily lose his class levels?

Ignoring the "If so" because this isn't actually related to the last one - not unless the class explicitly states they do. For an example, see the Paladin's Code of Conduct, which strips the Paladin of class features if it is broken (something a low Int character would almost certainly do, as an aside).

A character with 1-2 Int is, however, probably too stupid to make use of most of their class abilities. As with the last point, this is a pure roleplaying issue - someone with 1-2 Int would respond to situations like an animal. Lacking an understanding of long-term consequences or the ability to plan, they're unlikely to use complex skills of any kind without having been drilled to do so (e.g. a fighter might still make use of a weapon when attacking, but they're unlikely to fight with any finesse or much forethought).

What are the requirements for feats as well?

Presumably you're trying to ask whether or not they lose access to feats they already have, similar to your previous question about class abilities (from core book p112, emphasis mine):

Some feats have prerequisites. Your character must have the indicated ability score, class feature, feat, skill, base attack bonus, or other quality designated in order to select or use that feat

so the answer in this case is yes.

Would a PC be turned into a NPC?

Not as a necessary consequence of the rules; but if the loss of intelligence is likely to be long-term or permanent most people would choose to retire the character in most circumstances. Unless there's some kind of compelling reason to continue roleplaying the character beyond a last hurrah and sendoff, it's probably time for the GM to take over while the player starts rerolling. Of course if it's temporary intelligence damage the player should be roleplaying it - and if they suck at it or munchkin it up, then you might stop them for the sake of everyone else.

Answer 2

Not that I'm aware of for any base classes.

Some prestige classes might require it. And the general rule is that if you lose access to the prereq's of a class you lose access to the PRC's class features. But not Base attack Bonus or hit points.

If there's a minimum requirement for certain class features (IE: A spellcaster requiring a minimum intelligence of 12 to cast 2nd level spells won't be able to cast spells of lvl 2 or higher if he doesn't have at least intelligence 12.

Answer 3

Nope!

The only effect of having a mental stat at 1 or 2 is that you take a -4 or -5 penalty as your relevant attribute modifier, and don't qualify for certain special things that explicitly call out an intelligence requirement.

There are no effects, as far as I am aware, that turn a PC into an NPC in Pathfinder.

Some GMs use that as an excuse to explain why animals can't take class levels or unionize against their oppression, but this has absolutely no basis in the rules whatsoever. Even possibly intelligence-lacking possibly non-creature entities like natural unworked stone deposits have some form of personhood in Pathfinder, as evidenced by the Stonetell:

A stone's perspective, perception, and knowledge may prevent the stone from providing the details you are looking for.

Note that creatures with 0 Int, Wis, or Cha are effectively incapacitated while that stat remains 0.

Note that lacking a stat is different than having 0 in it, and does not apply a negative penalty to checks made with that stat. Lacking Charisma or Wisdom means you are an object, not a creature, though that doesn't affect whether you are a PC or not.

Lacking Intelligence means you possess the mindless special ability, and thus do not get skill points or feats and are immune to mind-affecting spells. It still doesn't prevent you from gaining class levels, talking, or engaging in activities or suffering from attacks that require or affect the mind, so long as they are not mind-affecting.

Having 2 or less intelligence and the Animal type severely restricts skill usages and what kinds of words people are allowed to use to describe the animal's actions, but this is mostly because the system hates animals, not because they have a low intelligence score; raising their intelligence above 3 just somewhat lessens the amount of jerk-GM-ing that is allowed to impact their skill usages.

But, basically, low mental scores above 0 have no special impact on gameplay.