This text is included in the help file for 'Spell Slots'
In addition, in order to cast a certain level of spell, the caster's primary
stat must equal 10 + the level of the spell. So, a 6th level wizard spell
would require and INT of 16 to cast.
However, numbers are not used in the game - only descriptions. While the above might be absolutely accurate, I am not even sure what my starting stats were. Since it requires at least a little planning to make sure your caster can utilize high-level spells, could the wording be altered to make it clearer just how intelligent/wise/charismatic a character has to be to use spells of a specific level?