I used the population of that age group as the denominator.
The numbers of infected by age group are not as easy to find, and perhaps not so accurate (confirmed vs non-confirmed, etc).
I've approached this numerically since numbers started coming out of NYC last spring. It's just the way I've been comfortable estimating/qualifying the risk.
I think that I see what you mean, and that sounds fair to me. Basically that the disease may have only moved through half that population so far, if I follow your logic.
The numbers of infected by age group are not as easy to find, and perhaps not so accurate (confirmed vs non-confirmed, etc).
I've approached this numerically since numbers started coming out of NYC last spring. It's just the way I've been comfortable estimating/qualifying the risk.