" Every language has grammar rules. They are in the minds of speakers of the language — including, for English, me. The fact that they weren't decided by an official edict doesn't mean these rules are a trivial matter; demanding people change their grammar rules is an affront. You might succeed in convincing me to change the English grammar rules in my mind, but don't you dare demand it. "
To more clearly answer your question -- it becomes correct when the corpus of the English speaking world decides it does. That hasn't happened since 1300, why would it happen today?