The median age, the age at which half its audience was younger and half older for viewers of the big four U.S. Television Networks is now 51, according to a report by veteran media analyst Steve Sternberg.

The report says the median age has climbed at twice the rate of the general U.S. population over the last twenty years.

The median age at CBS last season was 55 (up from 45 in 1991), ABC’s was at 51 (up from 37), NBC’s was 49 (up from 42) and Fox’s stood at 44 (up from 29). By comparison, the median age for the American population as a whole increased from 33 in 1990 to 38 last year, according to the U.S. Census Bureau.

The report means that more than half the audience at the major U.S. Networks is now above the advertiser-friendly 18-49 demo.

While some of the increase can be attributed to the graying of the population, a significant portion can be attributed to the type of shows airing on the network. By genre, comedies are the youngest skewing shows, while procedural dramas were among the oldest skewing genres. In the fall of 20099, the four networks aired 20 situation comedies and 20 procedural dramas, compared to 45 and 5 respectively in the fall of 1999.

It means that in order to reduce the median age of viewers networks are going to have to consider revamping their line-ups to include more situation comedies and less drama.

The oldest-skewing broadcast shows include CBS' "Sixty Minutes (60), "The Good Wife" (58) and "NCIS" franchise (57), and ABC's "Dancing With the Stars" (57). The youngest skewing shows were Fox's Sunday comedy block (30-32), “Glee" (38), and NBC's Thursday-night comedy block (35-40).

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(Edit: Corrected error regarding previous number of comedies and dramas.)