Given a diet of US imports such as Gossip Girl, Friends and Lost you'd be forgiven for thinking that most American television viewers were about 20. But figures this week have revealed that the average age of those watching TV in the US has tipped 50 for the first time.

A study of the big five broadcast networks from research firm Magna Global shows that the average viewer no longer falls within the coveted 18-49 demographic so sought after by advertisers. The median age of the population in the US is 38. For the 2007/08 seasons CBS attracted an average age of 54, ABC pulled in the 50-year-olds, NBC managed 49, Fox's viewers were a younger-still 44 and The CW (which airs Gossip Girl and Smallville) comes out at 34. If you factor in viewings on DVRs, the average age drops by a year in most cases, though that is still higher than it's ever been.

The youngest show on CBS was Big Brother (45) while the oldest was news and investigations flagship 60 Minutes (60). ABC varied from UK import Supernanny (41), to police drama Women's Murder Club (57); Fox swung from Family Guy and American Dad (29) to legal drama Canterbury's Law; and The CW from teen drama One Tree Hill (26) to family drama Life Is Wild (45). NBC's youngest-viewing show was Scrubs (34) and its oldest was the detective show Monk (58).

It doesn't take a detective to deduce that older viewers like crime dramas - which goes some way to explain the success of Monk, an obsessive-compulsive sleuth in San Francisco. The show has consistently pulled in high viewing figures - both on ABC where it originated and now on cable network USA - without either being a huge critical success or generating much buzz. That its average viewer is 58, making it the oldest-skewing non-news show out there, may go some way towards explaining the success of a show that got almost no press and never set internet chatrooms ablaze.

The cable networks vary more widely. Ultra-conservative Fox News maintains a median age of 65 across prime-time and daytime programming. The Golf Channel, other news networks and TV-movie showcase the Hallmark Channel are also greyer than average, while the kids' network Nickelodeon unsurprisingly mostly attracts 10-year-olds.

So where have the young people, except those kids watching SpongeBob Squarepants, gone? That the average viewing age drops when DVR figures are taken into account gives some indication that they are embracing new technology and setting their own schedules. Many also take advantage of streaming repeats online. The Hills, MTV's flagship reality show, got an average of 3.7 million viewers for new episodes - but factor in DVR figures, and you add another million. In addition, episodes and clips have been streamed 32m times online. In fact The CW stopped offering an online repeat for Gossip Girl in order to try to boost viewing figures. The network still, however, offers episodes for sale on iTunes, where the latest is regularly number one - perhaps because 12- to 17-year-olds are twice as likely to use Apple's media-buying hub as any other age group.

Those teens and tweens certainly represent those who might have sat down to watch telly before downloads became so easy. On the last numbers available, iTunes is responsible for about 50m TV and movie downloads a year. And that's just those buying legitimately. Studies show that a third of all internet traffic in the US comes through illegal filesharing.

That those choosing to switch on a TV rather than a computer are now older "makes network executives weep", says Cynthia Littleton, who covers TV for industry magazine Variety. She thinks there's now "a concerted push among networks and even the advertising community to break out of this ancient mindset that people over 50 are not such a good audience for commercials".

"Older viewers have a lot more disposable income than the 18-year-olds who seem to be the holy grail for the broadcast networks. In five to 10 years you'll see less of an intense focus on people from 18-34, and advertisers will be actively concentrating on 35-65," she adds. "Anything over 30 has this taint right now, like it's so old. But that will change."

Anyone who's seen the repeated adverts for Viagra (featuring a reworking of Elvis Presley's Viva Las Vegas as Viva Viagra), or a commercial for an osteoporosis pill that features a fiftysomething Sally Field, may see a difference in US TV already. But this change in thinking on target age groups may take a while to seep into commissioning - and thus imported shows in the UK.

British broadcasters may face similarly changing demographics. In the last UK census, the average age was 37.4 - making us only a few months younger than Americans. And, while figures are not available for the average viewing age of Britons, research by entertainment analysts Attentional shows that viewing time among those aged 16-34 has been declining faster than other age groups, while those over 55 now watch almost five hours a day.

Katy Bravery, editor of Saga magazine, thinks that the "tectonic force" of shifting demographics will force broadcasters to adapt here too. "It's not rocket science that programmers are simply playing to the market that's paying," she says.

· This article was amended on July 14 2008. In the article above we said that television show Monk - about a detective in San Francisco - lasted a season and a half before being dropped when the BBC bought it a couple of years ago. In fact Monk has been running on the BBC since 2003; the fifth season of the series on BBC2 has just finished. This has been corrected.