I can't speak for all of these sites. But, in general, you don't designate material as "18 and up" when you upload it.
Users have the ability to flag videos as "objectionable", "for mature viewers", etc.. If there are enough votes (how many, I don't know - and probably a trade secret) the video may get an "18 and up" warning, or may be removed.
I assume there is some manual review by management involved, once at least one user has flagged the video as objectionable.
It's really kind of unfortunate how this works. It's pretty-much a free-for-all with very broad interpretations of what's objectionable.
If you want to flag your own video as 18+ or 21+ because, for example, it promotes drinking alcohol or smoking - fergidaboutit! You're *way* too PC for the video sites! And dating? DATING? Even "dating". Naw.
This means that there are certain video that you probably *can't* upload without violating some law, since the sites don't allow you to flag content for age appropriateness, and that might be required by the law.
Keep in mind, though, that these sites aren't there as an advertising outlet for you - they are there to provide entertainment for their users. As such, you can't expect them to add features specifically geared to complying with advertising regulations.
Many of the sites do tolerate product-demo videos, etc. and they certainly are widely used for that purpose. But any advertising-related videos are there at the pleasure of the site, and might be taken-down any time if they crack down.