Sometimes this seems a little less than logical to me since the size of people of any given age can vary tremendously. What other criteria seems like they might be more useful for determining dosage of medicines?
There are many factors that would affect the dosage equation. For example, the size of the person is the most important, but there are other. The person's prior medical history and immune response are two large ones. A few subtle but important ones are BAC, gender, lactation, infection, dietary factors, and stress.