OSHA only requires that extinguishers be mounted, located, and identified so that they are readily accessible.
Mounting: The OSHA standard does not explicitly state how high a fire extinguisher should be mounted. However, OSHA publications indicate that that the handle of a fire extinguisher should be between 3.5 to 5 feet from the floor. However, OSHA can cite the General Duty Clause by referencing NFPA 10, “Standard for Portable Fire Extinguishers,” above if the height it is mounted could introduce a recognized hazard. Is that a bit of a stretch? Yes, but it could happen. NFPA 10 states that fire extinguishers should be no closer than 4 inches from the floor with the handle no higher than 5 feet, with the exception of 40 lb or larger extinguisher handles being no more than 3.5 feet from the floor.