1. Assuming there is a real proven age gap and balance bike allows a noticeably earlier start than a real bike with side wheels - what is the purpose of doing that in the first place? What will kid learn in the gap period specifically? Just balancing? Well, maybe that's beneficial.
I have googled and the quick search tells me that the gap is between 6 to 12 month.
2. I have never ever in my life seen even one balance bike with brakes. So as soon as a kid can "glide" on it, he is a danger to himself and to the people around.
Does learning balance a bit earlier outweigh the danger? I don't know.
As an adult I try to walk as far as possible from kids on these balance bikes, unless parent is holding their hand or handlebar. Getting hit at a significant speed in the leg is not fun.