Younger people are more likely to be watching TV on devices that actually have the ability to turn subtitles on. Closed captions are not the same as subtitles.

But the real reason this is happening is because it's impossible to hear what people are saying on TV. The audio mixing is horrible for TV sets. That's why my wife and I use them, at least. 

Younger people, in the 18-24 age group, are probably even more likely to be watching shows on tablets and phones, where the external speakers are abysmal. Even with headphones, phone-users are likely to be in noisier situations.

What's most telling about this is that the subtitles usage trends downward until the 65+ age group. That's to be expected, as some degree of hearing loss usually occurs with age.

The people in the 50-64 age group are probably better off financially than Millennials and Zoomers. They have sound systems in their homes, both because they have the money to buy them and the space to install them. I suspect the audio mixing on most shows isn't a problem with proper speakers.