harry potter - When is a "Wizard" confirmed as a Squib?

What is the defining traits of a Squib? In the books Neville himself (albeit early on) says

'They went for Filch first,' Neville said, his round face fearful, 'and everyone knows I'm almost a Squib.' -CoS

At what "magic level" is a wizard or witch confirmed as a Squib? Supposedly Filch is reading about Kwikspell in PS, but at what point does a wizard become a confirmed Squib (IE COMPLETELY unable to use magic, rather than just having weak or latent magical abilities)?

Is there a certain age at which magical abilities will no longer be considered "Possibly latent"?

I understand there's ways to tell the difference between a Squib and a Muggle, but that says nothing about Squibs vs "Late Bloomers".

Answer

It seems as though the process actually works the other way 'round: children are assumed to be Squibs first¹; so the question really is, when is a magical child definitively proven to be magical. The answer is kind of annoyingly unsatisfying:

As soon as they show strong accidental magic

The deciding factor, at least in Britain, seems to be Hogwarts' Book of Admittance and, in borderline cases, it errs on the side of "assume Squib until dramatically proven otherwise." The Pottermore article on the subject, for instance, remarks of Neville Longbottom:

A mere whiff of magic suffices for the Quill. The Book, however, will often snap shut, refusing to be written upon until it receives sufficiently dramatic evidence of magical ability.

Thus, the very moment that Neville Longbottom was born, the Quill attempted to write his name and was refused by the Book, which snapped shut. Even the midwife who attended Alice Longbottom had failed to notice that Neville managed to shift his blankets more snugly over himself moments after birth, assuming that his father had tucked the baby in more securely. Neville's family persistently missed faint signs of magic in him and not until he was eight years old did either his disappointed great aunts and uncles, or the old stickler of a Book, accept that he was truly a wizard, when he survived a fall that should have killed him.

This of course leads to the unfortunate circumstance where a child could actually be magical, but simply be so weakly magical that they are never admitted to Hogwarts. Presumably this is the niche that Kwikspell targets.

It's not clear how this might be different in other parts of the world.

---

¹ From the perspective of the Book of Admittance, anyway; I suspect a parent's expectations would be to assume magical, then gradually change to assume non-magical as no evidence of magical ability presents itself. In addition to what happened to Neville, this is largely what happened to Angus Buchanan, who was assumed to be magical by his parents, and got as far as the Hogwarts Sorting Hat (thanks to deception on the part of his siblings) before being "outed"