I very much believe that it depends on individual circumstance, too-- BUT-- I see an awful lot of parents locally doing it merely because of pop culture references to research (like in Outliers), and because everyone ELSE does it.

There are certainly children who aren't ready at five.

But then the question ought to be-- ready for WHAT?

No guarantee that such children will be MORE suited for it at 6 or 7, either, if you see what I mean.

Schrödinger's cat walks into a bar. And doesn't.