There is never going to be a one size fits all answer. It really depends on your child and the school environment. How big will the classes be? How supportive will the teachers be? It will also depend on your child - how will they react in various situations.
We started DS5 in our local GT private school at 4 and most likely his sister will follow at 4. But... his class has only 8-9 children and 2 teachers, and he was asking us to get him out of his daycare where he was so frustrated and unhappy so he was completely on board with the change. And he is one of those kids that just blends in with older and younger kids - he loves his classmates (and the girls especially look out for him). DD wants to go there herself as soon as she is able so again, that is something she has told us she wants.
If DS had resisted the idea of going to this school and was happy at his daycare, we would probably have waited another year ourselves (note, he misses the cutoff here for public school by just days, and we realized we could not wait *2* years for public school when he got so bored and angry about his daycare - which he used to love... until he realized he wanted more).
But... the school makeup is really important (teachers, admin, parents and kids). At DS's school, kids learn that everyone is to be respected regardless of age (we see mixed age groups play really well together and treat each other with respect), teachers are aware of development differences - and asynchrony of these kids - so it has worked really well for us. However, this is not the norm, as we seen with some limited exposure to other school environments.
