It would be quite hard.
A Youtube requires a immense database of video files, and huge bandwidth.
Current generation decentralized blockchain implementations couldn't handle that.
Honestly I think that it is not 'quite hard' but rather impossible with currently possible implementations and technology. In order for this to work and in order to access the content one would need to part of a P2P network (in order for it to be decentralized). In order for this to properly work there would need to be a lot of participants in addition to specialized software. Each participant would have to store parts of the content. Considering how much space good quality videos require (e.g. 4K) I doubt that more than a handful of people would be able to store them.
There are potentially other possible ways of doing this. However, I doubt that someone could get close to the exact definition of what Youtube is.
Here's a good example.