There are no international standard regulations for the size of a junior futsal ball and even in Spain until recently, they only used sizes 3 & 4.
They are also hard to get hold of, so in developing futsal countries, people revert to using the common size 4, even for children aged 5. While all balls can be useful for different outcomes, I think that kids should be using a ball appropriate to their size, so whenever I work with kids, I use the sizes in the table below, where possible.
I researched the regulations in Spain and Brazil for junior futsal ball size, so you can see the comparison and I think that the sizes I’ve recommended are suitable for the ages specified. From experience as well, these sizes work. I may use size 3 for under-13s, but as we tend to group under-13s with under-14s in England, then I have left it as size 4. If the grouping was under-13s and 12s together (like Brazil), I would copy the Brazilian sizes.
What size do you use in your country?