After decades chasing young viewers, the broadcast networks are starting to shift tactics — peppering their lineups with shows and actors who appeal to the growing audience of aging baby boomers.

“The Millers” on CBS features 72-year-old Beau Bridges and Margo Martindale, 62. NBC has James Spader, 54, in “The Blacklist” and is considering a new family sitcom with Bill Cosby, 76. ABC, stuck in last place, is developing a show about basketball buddies in their 60s.

The trend is being driven by demographics. Members of the baby boom generation will all be 50 or older this year, and they watch a disproportionate amount of TV.

The median age of a broadcast television viewer is now the highest ever at 54. Twenty years ago, it was 41. The most-watched scripted series in the 1993-94 season was “Home Improvement,” with a median viewer age of 34. Today, it’s “NCIS,” with a median viewer who is 61.

Confronted with these realities, the networks are aggressively making the case to advertisers that older viewers are valuable — especially the affluent and influential 55-to-64-year-olds they’re calling “alpha boomers.” The 50-and-up crowd of today, they contend, is far different than the frugal and brand-loyal group that came of age during the Great Depression and World War II.

“These people are more active, healthier and much more likely to still be in the workforce,” said David Poltrack, chief research officer at CBS. “It’s certainly a much more vibrant and economically active audience than it used to be.”

Ironically, it was the baby boomers who first led TV networks to cater to younger viewers in the 1960s.

Until then, network ad rates were based simply on the number of households watching. But as the first wave of Americans born after World War II entered adulthood, third-place network ABC saw an opportunity to promote this audience as super consumers and pushed for advertisers to focus on viewers younger than 50.

By the 1970s, ABC was booming with youth-oriented hits such as “Charlie’s Angels” and “Happy Days.”

Now CBS, NBC and ABC are pointing to research that shows consumers over 50 spend some $90 billion a year on cars alone. By 2017, they will control 70 percent of the nation’s disposable income.

“Boomers have been marketed to since they were 5, because they were the first generation raised in front of the TV,” said Matt Thornhill, founder of the marketing think tank the Boomer Project.

In contrast, their children, known variously as millennials, Gen Y or echo boomers, are living at home longer and making fewer key purchasing decisions than their predecessors.

In theory, that should make them less attractive to advertisers. But in reality, they are more prized than ever.

Younger adults, busy with school, careers, socializing and child-rearing, have always watched less TV. Now they’re even harder to find, thanks to technology that allows them to watch TV whenever and wherever they want and to skip over commercials.

Since these viewers are so elusive, advertisers will pay more to reach them.

“Television naturally skews older, and that’s why advertisers have historically paid a premium for a younger audience,” said Neil Ascher, managing director of Zenith Optimedia, which advises companies that buy TV advertising.

Network ad sales executives are working to change those calibrations, but it’s hard to quantify their success. Buying time on network television is a complicated series of negotiations and there is no industry-wide rate standard.

A decade ago, networks primarily sold ad inventory for prime-time shows based on how many 18-to-49-year-olds were watching at home. Now, network sales teams are emphasizing other metrics, such as income and education level of viewers.

For example, ads on CBS’ “The Good Wife,” ABC’s “Modern Family,” and NBC’s “Parks and Recreation” command a premium because they draw more affluent viewers, industry experts say.

“Everyone over 50 has essentially been given away for free to advertisers,” said Jack MacKenzie, president of Magid Generational Strategies, a consulting firm that advises Fortune 500 companies. “What we’re telling our media clients is you’ve got to find a way not to give them away anymore.”

Executives at NBC, CBS and ABC are especially keen on the tech-savvy alpha boomers, according to Alan Wurtzel, president of research and media development at NBC Universal.

As consumers, they spend considerably more on home improvement, credit cards, travel and beauty products than do younger cohorts. Their image-consciousness makes them ideal consumers, said Wurtzel: “If alpha boomers aren’t vain, I don’t know who is.”