What specifically do you need to know?
The decrease for the various age groups can be found in the census links I provided earlier. The only information at this time would be to take the 2010 and 2000 census numbers and look at the change in each. Census breaks it down by 0-4 and then 5 on. You can see the change in each county as most had a decrease except for one I believe.
Afraid finding numbers in that age group is difficult at best. Taking the census numbers would be as close as possible to find the change in the population in this age group. Hope then you can use some of that information as well as I provided the square miles you asked for in the various counties you requested. I will then opt-out for other expert to possibly assist.