There are very few videos with age restriction and most of them have to do with sexually suggestive content, which doesn't make much sense because teenagers (at least male teens) consume porn anyways.
What kind of unrestricted videos are inappropiate for teens?
Youtube expects you to rate your videos yourself, or you can get reported and they will do it for you. Basically content that contains one of the following is supposed to be 18+ [1]:
* Vulgar language
* Violence and disturbing imagery
* Nudity and sexually suggestive content
* Portrayal of harmful or dangerous activities
Personally I put restriction when I upload game videos with graphic violence as that is not really suited for non adults imho. I'm less likely to put restriction on nudity and vulgar language although I don't swear myself in my videos.
The question wasn't about how to filter to teenagers, was it?
The question was about what people might want to filter to teenagers.
I guess what I'm saying is that the premise is false.
Most societies have an expectation that some content isn't shown to, for example a 13 year old person. In the US you have content ratings like NC-17, don't you? In the UK we have 15 and 18 rated content. So I guess content like that, some people would filter for teenagers.