Like I said, you can do this calculation (percentage infected) for an age bracket (#infected in age bracket / population size in age bracket) or you can do it for the whole population (#infected in whole population / whole population size).

But it doesn't make sense to do #infected in age bracket / whole population size. You might as well calculate #infected in age bracket / number of cattle in Texas.

Of course there are many reasons why one would want to look at the number of infected in certain age (or social or whatever) groups. Beyond that, I'd prefer not to get further into the weeds with you.