There are more old people watching TV than young people
Young people are watching TV but in nontraditional ways that are hard to monetize. Old coots aren't changing their habits, but they are moving out of the 18-49 age group advertisers care about reaching.
This is a business problem that all the networks are going to have to figure out how to solve. NBC and CW, you're up to bat first. Then ABC and FOX. CBS bats last. But it's everyone's problem. The irony is that NBC and CW could benefit from being forced to confront this problem early, and CBS could end up regretting their complacency and inaction.
NBC should go after the older audeince
Oooooh no. Not as long as advertisers cling to their archaic notion that people over 49 are worthless to reach because their brand loyalties are set in stone. (They are most certainly NOT.) A 49 year old Baby Boomer is not the same sort of person as a 49 year old who was born during the Depression, and does not have any more brand loyalty than any of us do, but just try to tell advertisers that.
What is wrong
with advertisers? They think all people over 49 are perpetual Ma Joads? As long as they are ruled by their misperceptions, the networks are forced to warp their TV lineups to suit.