The networks still preach adults 18-49, but the Big Three are all expected to post median ages above 50 this fall — with Fox not too far behind.

According to a recent study by former Magna Global EVP Steve Sternberg, the broadcast networks as a whole have once again grown older than ever. The five broadcast nets’ average live median age this year — in other words, not counting DVR usage — was 51.

That’s a whopping 8-year uptick from 10 years ago, when the nets’ median age was 43. In comparison, the median age of TV households has grown much less from 1998-1999 to 2008-2009, to 38 from 36.

“While CBS’ average median age remains over 50, ABC has aged up to hit 50 for the first time, and NBC has been hovering close to 50 for a few years now,” wrote Sternberg, in his final report for Magna (which he departed earlier this summer).

“And with Leno taking over the Monday-Friday 10 p.m. time slots next season, it will be hard-pressed not to surpass 50 as well,” he added.

For the just-completed 2008-2009 TV season, CBS was oldest with live viewing, hitting 55 — the first time it had traveled out of even the 25-54 demo. (When DVR usage is counted, that number goes back down to 54). ABC was next, at 51 (and 50 with DVR), followed by NBC (49, or 47 with DVR), Fox (46, or 44 with DVR) and the CW (34, or 33 with DVR).

In the case of those live viewing median ages, they all rep the oldest any of those nets has been in their respective histories.

That’s because the broadcasters began to greatly age themselves a decade ago as they mostly got out of the kids and teens business. With few shows at the major broadcasters targeted at that aud, save reality competitions like “American Idol,” those viewers fled.

What’s more, the rise in procedural dramas — which are like catnip to older viewers — and the collapse of comedy — which traditionally attracts younger eyeballs — help cement the graying of the nets. Ditto the multiple hours of older-skewing reality and gameshows such as “Dancing With the Stars” and “Deal or No Deal,” as well as the use of moldy repeats on Saturday nights.

Fox, in particular, has gotten quite a bit older over the past few years, as the percentage of its audience that are teenagers has dipped from 12% in 2004 to just 8% in 2008. Conversely, the chunk of Fox’s audience that is between 50 and 64 has risen from 18% in 2004 to 26% in 2008.

On the cable side, major nets haven’t experienced the same problem — and some have actually seen their median ages decline in recent years. TBS, for example, sports a median age of 35, unchanged from last year and down from 38 just two years ago. Sister TNT’s primetime median age has slipped to 44, from 46, while USA’s primetime median age has held steady at 46 for years. FX also held steady, at 37.

The oldest-skewing cabler remains Fox News, with a median age of over 65, followed by CNBC, GSN, Hallmark and Golf Channel. Noggin (6) is youngest, then Nickelodeon and Cartoon Network.

At ABC, the youngest-skewing show was “Scrubs,” with a median age of 39, while “Dancing With the Stars” was oldest, at 56. CBS’ youngest was “How I Met Your Mother,” with a median of 45, while “60 Minutes” was oldest (60).

“The Office” was youngest at NBC, with a median age of 35, while “Law and Order: Criminal Intent” was oldest, at 59. At Fox, “Family Guy” continued to be the youngest (29), with “Are You Smarter Than a 5th Grader” the oldest (57). And at CW, “One Tree Hill” was youngest in median age (27), and “Supernatural” was oldest (39).

If there was any silver lining for the nets, it’s that DVR usage attracted a much younger audience, helping temper their shows’ median ages somewhat. According to Sternberg, even though the nets’ live median age now averages out at 51, their combined median age plummets to just 40 during DVR playback.

The DVR audience for ABC, CBS and NBC is 10 years younger, while it’s seven years younger for Fox and two years younger for CW. For example, the median age for the audience watching ABC’s “Grey’s Anatomy” live is 48.9, while the audience watching the show on DVR has a median age of 38.3. At CBS, the live audience watching “Amazing Race” has a median age of 51.9, while the aud watching the show while time-shifting is aged just 39.2.

Sternberg, who began producing median age reports more than 15 years ago, said roughly 14% of total viewership is now time-shifted, although it’s as much as 30% for shows such as “Lost,” “The Office,” “Heroes,” “Dollhouse” and “90210.”