My 8 yo dd definitely visualizes, and has for at least a few years that I am aware of, so I do not think age is a factor. I learned the art of visualization from my gym coach back in the 80's- he would lead the entire team on "guided meditations." I have often used this technique with both of my daughters for various things. The key is to be relaxed and completely present. Focus on breathing, works best when lying down and eyes closed. You can have silence or soothing music in the background and ask your dd in a calm- almost monotone voice to start with her toes- become aware of them and aware of the tension leaving them and then work up the body, one section at a time until you come to their head. The whole while focus on breathing. Then guide them through each event. It helps if you know their routines, but if not, begin with , for example, bars- putting on grips, getting chalk, breathing, knowing she is going to execute a flawless routine, preparing for mount, focusing on how she grasps and swings and releases and is powerful and fearless and tight and graceful swinging between bars, setting up for dismount, and sticking it. Continue like this through the events. Also, teach your dd that if there are certain words or phrases the coach says for certain moves, to say them to herself while visualizing.
Learning how to be present and relaxed is the biggie. Yoga and other forms of meditation are wonderful assets for athletes, and they really help pave the way for the skills needed to visualize. It is just like dreaming, but with utmost confidence and perfection- there are no uncontrolled variables. Your vision is what you alone create. LOTS of positivity!
Good luck to all of your dds on their "vision quest!"