Their ratings are still strong, and their revenue is still strong, but according to this piece in the New York Times, the median age of their viewers is now over 65 years old. This is the way the NYT describes the audience...
Just how old is its audience? It is impossible to be precise because Nielsen stops giving an exact figure for median age once it passes 65. But for six of the last eight years, Fox News has had a median age of 65-plus and the number of viewers in the 25-54 year old group has been falling consistently, down five years in a row in prime time, from an average of 557,000 viewers five years ago to 379,000 this year. That has occurred even though Fox’s overall audience in prime time is up this year, to 2.02 million from 1.89 million three years ago.
Of course, if you're a Fox News viewer, you just pshaw, and say "liberal media" when you hear that this story was in the New York Times.