As age groups get vaccinated and have a corresponding drop in infections, then, the groups with lesser (first dose) or no vaccinations, will, mathematically, have a greater volume of cases for their populations size. In other words, if we ever got to everyone 100% vaccinated, under than under 12, and if you could say 100% vaccinated meant that you would have no infections, then, ALL cases would be for under 12. (This, i believe, is highly improbable, but, it speaks to why trends need to take into account all factors.