Almost half of adults 22 to 45 years old are watching absolutely no content on traditional TV platforms, according to a new study by Omnicom Media Group agency Hearts & Science.

Instead, this 47% is consuming TV content and video on streaming platforms that didn't exist as recently as the series premiere of CBS's "NCIS." That doesn't mean they aren't watching TV content or even that they aren't seeing ads. They're just consuming it in places where ad models vary, audiences are fragmented and measurement is harder.

"It's pretty scary," says Hearts & Science CEO Scott Hagedorn, referring to the group as "unreachable" by marketers. "We are not reaching young audiences effectively, just over-indexing on older viewers on TV."

TV viewing for decades was described by Nielsen, but the proliferation of new platforms, viewing habits, devices and ad models has complicated everything considerably. While Nielsen has the technology to measure many of these non-traditional platforms, roll-out has been hampered by the heavy lift it requires from TV networks and other content providers.

Less than one-third of the TV and video watched by millennials and Gen Xers is accounted for by traditional measurements, according to the study, which included a survey of about 1,500 people. The rest is being watched in apps on smartphones and streaming devices like Roku and Apple TV.

Marketers have to think creatively not only to reach millennials and Gen Xers but also to make the most of it when they do, according to Hagedorn. "We are headed toward a creative wake-up call," he says. "If most of this content is being consumed in-app, we need to think more about the utility of advertising here."