5 - 8. I was concerned at first. But there's so many of these on YouTube that have pretty high view counts, that my worry pretty quickly went from "my friend's kids are weird" to "kids are weird".
I used to think it was disconcerting, but I also had peers that did the same and turned out alright, so I wonder if it's part of figuring out stimulus/work/gratification/reward/effort as a child and eventually growing into an adult.