Earl of Cornbread
Yeah, languages change. Hence, it's grammatically correct.It has been used for 700 years, but that doesn't make it grammatically correct. ..
That all said, it's changed in how it's been used (as language is wont to do), ...
And for the citation-minded ParanoydStyle in Comedy of Errors, Act IV, Scene 3:
ANTIPHOLUS OF SYRACUSE
There’s not a man I meet but doth salute me
As if I were their well-acquainted friend,
The antecedent of the bolded their is singular "man".