"Young gamers" appears to be adolescents who play games between 11-16 years of age, with a second survey sighted targeting feedback from "young people" ages 13-24. They have more specific details at the PDF link.
Ideally we'd have access to the long paper (this is only the short paper) to get more questions answered. But there's no reason to assume their methodology is particularly problematic just because you haven't been granted access to the full breath of it.
Don't miss the forest for the trees.
not understanding the recruiting methodology is actually pretty important if you're going to use an non-ubiquitous label like "gamer" without clearly defining what that actually means. Just because you're willing to let your confirmation bias consume this report without any scrutiny doesn't mean the rest of us have to.