Because they combine children and adolescents (1-19). In their 2018 publication [0], you can see they actually do distinguish between younger children (1-4), older children (5-9), and adolescents (10-19). Your complaint was that 19 is an arbitrary cutoff, or even in bad faith, but it’s actually a common established definition in health contexts.

[0] https://www.nejm.org/doi/full/10.1056/NEJMsr1804754