Last week, I said “young viewers just don’t watch The O’Reilly Factor like they used to.” It turns out the same thing can be said for all of Fox News Channel.

April marked FNC’s lowest-rated month in the 25-54 demographic since August 2001, according to Nielsen data.

Caveat: 2006 hasn’t been as “newsy” a year as 2005, so far. But the trend of demo decreases isn’t just a year-over-year story.

In total day, FNC averaged 208,000 demo viewers in April, down from 212,000 in March and 236,000 in February. April’s viewership was at its lowest level since 9/11.

In total day, FNC averaged 402,000 demo viewers in 2003, 323,000 in 2004, 300,000 in 2005, and just 221,000 so far in 2006. (Year-to-date, FNC is down 26 percent from 2005, while CNN is down 21 percent.)

In primetime, FNC averaged 317,000 demo viewers in April, down from 321,000 in March and 344,000 in February. January (319,000) was the network’s second-lowest month since 9/11. (Click on the graph to see it full-size.)

After Sept. 11, 2001, many new viewers discovered cable news, and ratings rose across the board. Among total viewers, FNC is holding steady: The network had 1,484,000 viewers in primetime in Sept. 2001, and 1,440,000 in primetime in April 2006. But among the highly desirable viewers in the 25-54 demo, FNC is slipping. The net averaged 623,000 demo viewers in prime in Sept. 2001, but was down to just 317,000 in April 2006.

FNC averaged 609,000 demo viewers in 2003, 530,000 in 2004, 462,000 in 2005, and just 319,000 so far in 2006. But this isn’t meant to be a history lesson. FNC is recording its lowest ratings in almost five years in all the demos. April’s 236,000 in the 18-49 demo was the worst since 9/11. And April’s 58,000 was the second-worst in the 18-34 demo. (The worst came in March with 57,000.)

FNC isn’t alone in its demo woes. April was CNN’s lowest month for primetime demo viewers since December 2002. But by virtue of being #1, FNC has the most to lose.