I am talking about the ages, not how they view. There are overnight ratings, mostly live or immediate playback on a VTR. There are 24 hour ratings that add in "next day" and even 3-day summaries that include live and recorded.Forgive my ignorance but when you say networks target ages 18-49... I suppose you mean they targer mostly on-demand viewers or something like that. I can't imagine the 18-25 crowd actually sitting down in front of the TV to watch a CBS sitcom. And this comes from someone who is well in supposed target age of the networks.
However a show is viewed, networks target 18-49 because that is what ad agencies look at... either all of it or some subset like "English dominant Hispanic women 25-44" and the like.
We are talking here about broadcast nets ABC, CBS, NBC, Fox, Univision, Telemundo and the CW.