A lot of viewers of minecraft videos are under the age of 15. Many of them are as young as five or six. Minecraft is a very popular game with that demographic, and the majority of YouTube viewership of minecraft videos is children who like playing and also like watching along with other players. Watch someone like StampyCat and it becomes very clear that it's basically children's programming that happens to be very cheap to produce and easy for kids to access.
There is also a whole other world of people who mod Minecraft into various complex experiences exploring everything from computer programming to quantum physics, and that is a huge genre, but most Minecraft video views are kids.