No official answer, up to the DM, as others have said.
To me personally, psionics represents something that is strictly internal and is a property of having a thinking mind. Anyone in the fantasy world could see the future, if they knew how to look. This provides a contrast to magic, which is sourced externally. Maybe you get it by waving your hands around a particular way, or maybe you beg your god for it, but it's not a property of your character, and not necessarily anyone can have it.
With that in mind, to me anyone can (and should) learn how to use 100% of their brain, which in the game means using it to disintegrate things and read minds.