I am confused by your question.
If you have two 6 year olds per 100 6 year olds, two 7 year olds per 100 7 year olds, etc, up to two 17 year olds per 100 17 year olds, you *also* have two 6-17 year olds per 100 6-17 year olds (there are 24 6-17 year olds in 1200 6-17 year olds, which is the same as two in 100).
How is this different from having two per 100 18-100 year olds? For every 50 people in this age group, there would one, for every 100 there would be two, for every 1000 there would be 20, and so on.
But maybe I'm missing something.
