The hottest shows on TV networks — which command the highest ad prices — are attracting older viewers, which is a challenge for brands that want to reach millennials and teens. For instance, this season’s top-rated show, the revival of “Roseanne,” has a median viewer age of 52.9 years. The network show with the lowest median age is “Riverdale” on the CW, at 37.2.

Google’s YouTube, on the other hand, is wildly popular with much younger viewers. And the brands are so eager to reach those viewers that they have been willing to continue advertising on YouTube despite the issues it has faced around ads showing up on offensive content, like racist videos.

As TV ad spending has begun to drop, marketers have been diverting more money to tech giants like Google and Facebook, which have increasingly focused on expanding their video — and video ad — business.