As a parent I frequently discuss them with my Youtube obsessed child, not only in terms of age appropriate content but also to educate them, as a wannabe youtuber themselves, on what it could mean for them and if it is a road they wish to travel.
This includes what information to share and what not to share in terms of personal and appropriate information, the reality that not everyone in the internet is a nice person and youtube especially is rife with trolls and manipulators and most importantly building a relationship of trust with my child wherein they will be open with me about the content they watch and perhaps in the future share with the internet themselves.
ET Actually answer your question, the right time is now but be tactful about it. As others have said offering to moderate the social media aspect of it could save your nephew and family a lot of stress if they are new and naive to this kind of thing.