Notice that not everyone subscribes to the Critical Period Hypothesis.
No robust empirical evidence supporting various CPH theories is available at this point.
Here's a very good quote from an excellent textbook written by Lourdes Ortega:
"An early start does not guarantee complete and successful acquisition in all cases, as some children who start learning the L2 at an age as early as four or even two may be found to differ from native speaker performance in subtle ways" (Ortega 2009, p. 29).
See chapter 2 with an excellent summary and annotated suggestions for further reading in Ortega, L. 2009. Understanding second language acquisition. London: Hodder Education.
Also, see a recent monograph by Herschensohn, Julia. 2007. Language development and age. Cambridge: Cambridge University Press.