We did it with an 8 year old, around 48 inches tall, their main concern was the weight of the vest you wear and the goggles/headset might slip down on a really small head. She was able to handle it fine, plus it was awesome seeing an extremely short storm trooper in the game. A nine year old will have no problem at all especially at 54in, no worries about being turned away. A member of staff is in the room(s) the entire time monitoring and helped our daughter once when her mask slid down during the experience. Definitely do it it is worth it.
The only reason I can see why they worded it "strongly recommend" is to ward off people sending in kids 5-8 years old in and then complaining, wanting refunds because the gear doesn't stay on them, headset slides down, and ruining the experience for the group.