The data referenced in that quote is linked in the article. When following the link, it specifies "Americans under the age of 18"
Full info: The Gun Violence Archive
Americans under the age of 18 killed or injured by gunfire since 2014:
2014: 2,861
2015: 3,378
2016: 3,820
2017: 3,982
2018: 3,543
2019: 3,825
2020: 5,159
2021: 5,705
2022: 6,170
2023: 4,319 [in 235 days]
I'd love to discuss this more if you'd be interested. I teach mass communications and visual literacy courses at a university and have over a decade of experience. I'm always interested in discussing these things further, as media literacy is extremely important and we can only educate others and make improvements with our literacy skills through discussion, learning, and practice.
Could you tell me more about the authors you mention and where you got the information from?
One thing I like about Media Bias Fact Check is that their methodology is transparent and clear. Yes, there is a certain level of subjectivity, as there is with any analysis like this. They utilize fact checking best practices and have ethical funding. Even their competitors rate them to be accurate and credible.
They are considered reliable not because people use them, but because of their methodologies, transparency, and factuality. Nothing is 100%, but it's a good resource in my opinion.