It's a little sloppy the way it's written out, but I think you're the one who misunderstood it. The way I read it as:
The landmark Pfizer-BioNTech Phase 3 clinical trial began in late July 2020, recruiting participants aged 12 and over. Close to three thousand participants were adolescents: 2,259 participants were between the ages of 12-15 and 754 participants were 16 and 17 years old.
In March 2021, Pfizer and BioNTech dosed the first healthy children in a global Phase 1/2/3 continuous study to learn if the vaccine can produce an immune response against COVID-19, and if it is safe, in children aged 6 months to 11 years.
July 2020: 12-17 year olds
March 2021: 6 months to 11 years