Heather-Lee Mainville MabryI remember when I studied a little
   linguistics in college and the prof showed a mind-blowing graph
   of critical period of language acquisition (I think its' still
   the current model). From 0-4 years old was almost a straight
   line, going down a tiny bit. There was STEEP drop down to almost
   zero from 4-12, then 13-death was almost Flatline. "