Suddenly the audience watching broadcast television is looking a lot older.

A new study reports that when it comes to live viewing--folks watching shows as they air--the median viewer age has climbed to 50. That's one year older than the top end of the demographic most targeted by advertisers, adults 18-49.

The median age of the broadcast audience has been trending up for years. What's kicked it up is the increasing use of DVRs, which are favored by younger viewers. Fewer young people are watching shows as they air, lifting the median age of the network’s live-only audience.

This is a problem for advertisers on two levels. One of course is the aging of the live viewing audience.

The other is that those younger DVR viewers have the ability to zap through ads on the shows they've recorded for playback. Advertisers don't know whether their ads are being seen or not.

"It’s getting tougher to reach 18-24 year olds," says Steve Sternberg, executive vice president of audience analysis at Magna Global, which did the study.

The median age of CBS’s audience on this measure is 54 years old. For ABC it’s 50 and for NBC it’s 49 years old. The median age of Fox’s audience is 44 and the CW’s is 34.

While nearly one-fourth of homes have a DVR, only 9 percent of TV viewing is of programs recorded on DVRs and viewed after the original broadcast.

But with young people far more inclined to record programs, the shows they favor, like NBC’s "Heroes," can draw as much as 20 percent of their audience after the initial airing.

These viewers are often 10 or more years younger than the audience watching the original broadcast.

Younger viewers who record shows on DVRs are counted in the ratings media buyers use to negotiate ad rates with the networks. But it’s still unclear how many of these viewers are watching commercials in playback or fast-forwarding through ads.

Some recent reports suggest that people watching programming in playback may pay more attention to these programs and the commercials than to live programming. But Sternberg says a lot more research needs to be conducted to see if this is correct or not.

The worry for the broadcast networks is that advertisers will shift their spending to alternatives where they know they'll be reaching younger consumers.

Among the options are new media outlets, young-skewing cable networks, the low-rated but young-skewing broadcast network CW and Spanish-language TV, which has a median age of roughly 34 years old.

The Big Four may increasingly tout their 25-54 audiences and, if that happens, the type of commercials airing on broadcast may reflect this shift, such as more pharmaceutical ads popping up.

"There are still numerous shows on broadcast that skew younger," says Sternberg. "But, certainly, there are many young-skewing vehicles throughout the television landscape on cable, syndication and Spanish-language networks."