All age groups are equally INFECTIOUS once they have the disease, but are not equally able to BE INFECTED , otherwise people under 55 wouldn't have to wear masks. We wear the mask to protect others right? No matter what our age is.
So why would you reduce the population size to just an age slice?
The same assumption is made when calculating herd immunity. They use the entire populations infectiousness because chance to transmit doesn't change with age but chance to be infected does..
But you DO limit by age group on the infection side because that individuals chance of contracting it from the general population, is based on age/immune system .