A player character may equipment any weapons or armor that they so choose, however unless that character is proficient with a given weapon and/or armor type they will suffer Combat Penalties.
Each character class has a Natural Proficiency with several weapon and armor types upon character creation. Classes also have a selection of Trainable Proficiency’s that can be learned at any time. There are no specific level or title requirements for training in new weapon and armor types, however there is typically a gold and training time requirement (see Shops: Trainers).
Natural and Trainable Proficiency can be found in the associated Class Codex for a player character. Any weapon or armor type that is not listed as a Proficiency can never be used without suffering Combat Penalties (i.e. a Mage can never wear Plate armor without suffer penalties).