Kid: I'd say the lower bound is around 5, and the upper bound is variable depending on the individual...

In this case, I'd suggest the upper bound doesn't matter, as the criteria for filtering should be "a semi-unattended 5 year old could view it without concern."

All your examples are of topics where it's probably best for parents to initiate their child's education on the topic rather than Youtube randomly putting it in some kid's feed.