I think four for scratch junior with help, and then make code at six? He wasn't able to use either of them independently at first, it's a learning process.

But it's different for every kid! He was a very early reader and he's autistic, one of the markers of his presentation of which is being ahead of some developmental benchmarks and behind on others. If you have the resources, those kids coding clubs are all over these days and they do a great job of making it a fun, social activity for a certain type of dorky kid.