Update: I've just checked I'm right about this so I've got a couple of extra things to add:I have a useful tip if you use Duckduckgo as your search engine.
If you search for a Youtube video in Duckduck go under the 'Videos' section, then you will be presented with a small screen that explains that Google don't allow you to connect to Youtube anonymously and do you want to view the video here?
If you view it on Duckduck go then it doesn't play any ads- well at least I've never seen any but I'm not a huge Youtube user, maybe there are some eventually.
There is still a setting to play full screen so I don't see a down side.
If you click on the thumbnail you get the prompt, if you click on the link you go to Youtube
You can change the setting under 'privacy' to always play in Duckduckgo rather than be prompted.