The source for this is the CDC, which groups ages 1 - 17 as children, and 18+ as an adult. Johns Hopkins collected the gun violence data into a nice infographic. You can read it here: chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://publichealth.jhu.edu/sites/default/files/2024-09/2022-cgvs-gun-violence-in-the-united-states.pdf
In their introduction, they exclude 18-19 year olds from any data so they can be certain it focuses on chilren.