On the PowerLung website, at the bottom of each PowerLung model product page is the “Recommended Age” for that model. With a child it is always best to err on the side of lower resistance as they are still growing.
The real focus at younger ages should be good deep breathing form as strengthening core muscles. This is a key component for running and exercise in general. This will also be a good foundation for any activity your child chooses in athletic areas or performing arts.
While PowerLung provides a training program, when you consult your physician or healthcare professional they may recommend a different program, routine or method. Please be sure to follow their instructions. If your child feels any discomfort or pain you should stop immediately and contact your healthcare professional.