What constitutes the determination of a normal blood glucose level?
Determining what is considered to be "normal" is quite challenging indeed. Both the ADA and the WHO look at upgrading diagnosis limits and both have a conservative approach, primarily due to the difficulties in conducting large scale studies and the anomalies between assumptions of plasma glucose distribution and what is probably actually present in populations. Digging through, I found a very good WHO document here:

http://www.who.int/diabetes/publications/Definition%20and%20diagnosis%20of%20diabetes_new.pdf

I am not going to regurgitate from this document, but I believe Issue 2 provides a detailed answer to your question detailing several determination methodologies. 

As to your second question on age/sex variations, are you asking if the criteria are different? At least with reference to FPG, they don't appear to be so.