The demographics are only reported when the system is 99% sure of the users gender or age..... this is determined by user data, because of the the number of members, and the typical audience of Yahoo search and Yahoo owned and operated sites are our members, ads can be delivered based upon demo targeting by their registration information. Again though ads are only served on demo targeted campaigns if the system is 99% sure oi the end user demographics match your targeting.
I hope this helps to answer your question...