Butterfly knives _are_ cool, and a lot of fun! Of course a 12–year–old is going to ask for one; all of them do. We all wanted butterfly knives decades before Youtube existed so clearly this is not something to can blame Youtube for. I have a sneaking suspicion that our parents wanted butterfly knives when they were our age, even though they acted like it was a terrible idea when we asked for them.

But certainly a person’s first knife should be rather more utilitarian than that. It should be a size suited to their hand, it should be a single–edged knife intended as a practical tool, and you should be able to teach them how to use it effectively and safely to accomplish useful tasks. A simple pocket knife is probably the best choice.