What age does a staff head split?

It doesn't. Heads do not split, or brains fall out.

What you're referring to is the extra muscle that grows at sexual maturity, which can be from 6 months of age in a staffy, but more commonly from 9 months of age. The muscle on the head is one of the secondary sexual characteristics of the male.

I think this myth comes from the fact that the muscle grows in two bulges either side of the head, causing the appearance of a dent on top of the head. Anyone who stops and thinks for a second will realise that a split head would be a skull fracture, which is not a natural part of growth.