I just asked this question to a friend who used to work at Nielsen. I'm waiting for them to reply. I don't get how they do the demo breakdown either other than having the Nielsen families keep their own log of some sort. If you have a 49 year old mother, a 50 year old father, an 18 year old child, and a 16 year old child how do you tell who is watching TV during a given 15 minute increment?
My guess is they don't and if the TV is on a certain channel in that house hold then each demo represented by that household gets "credit". Since each household is measured using this same imperfect method it probably all balances out statistically somehow in the end. I'm sure these market research firms have some mathmatical way to account for it.