I emailed a company that does Grand Canyon mule tours and got the following reply
Rider must be at least 9 years old
Riders must be in good physical condition
Riders must weigh less than 200 pounds (90 kg), fully dressed
Riders must be at least 4 feet, 9 inches tall (1.44 meters)
Each rider must be able to speak and understand fluent English
Riders should not be afraid of heights or large animals
Riders age 17 and under must be accompanied by an adult 18 or older
Long sleeve shirts, long pants, broad brim hats and close-toed shoes are required
Ski caps and gloves are recommended for winter riders
Rider’s height must be proportionate to his or her weight