You would think that Youtube would realize that there are audiences of all ages and how much like channels on a TV, there are some that aren't exactly aimed at children. It would be like saying that History channel can't air any content from WW2 because it can be too graphic for kids. A ridiculous example of course, but that is how some are viewing this whole mess right now.
Except the way Youtube doles out advertisements and the way TV does so is so inherently different that any comparisons are moot. In TV the network has a certain number of ad-slots for every show it airs, and then, because of the decreased amount of content, it sells those slots off to advertisers on a case-by-case basis. So, to use your WW2 example, a mature documentary for WW2 might air adverts for more adult-orientated products such as cars or insurance (to put it simply); whereas if the history channel were airing a light-hearted show aimed at kids it would sell its slots to toy companies and the like. Another example on the other side might be Nintendo who, during the Superbowl, aired an advert for Zelda whereas if they were to buy a slot on Cartoon Network it would be for Splatoon 2 or Mario Kart instead.
However, with Youtube, advertisers say what demographics they want to market to alongside some other basic and easy-to-avoid descriptors and then
that's it, what videos their adverts fall on are left in the digital hands of Youtube's algorithms. The sheer level of content, even monetized, is so large that it's financially impossible for either Google or advertisers to do things like they do with TV, Cinema, etc. So, potentially, someone who wants to market to Males Aged 20-30 with an interest in video-games may end up with their videos on a racist tirade about how Mafia 3 had a black protagonist. Or, as the lines blur between what demographics watch, an alcohol company could end up with their adverts being shown to minors which would a PR nightmare if found out. Only now they're starting to add in what they don't want to have their videos put on, so to re-use the Nintendo example above, if they're looking to put Splatoon 2 adverts on Youtube they might not want those be seen on videos which feature violence of profanity so they blacklist them.