What neurobiological changes are associated with the end of the critical period in children? Developing children are in a critical period during which they are much better at learning certain tasks like speaking a language. After the critical period ends, children have a qualitatively more difficult time learning these tasks.

What specific changes in a child's brain are responsible for the drastic difference in learning during and after the critical period? Are these changes just to the plasticity and rate of neuorogenesis? 

Notes


The best understood critical period is phoneme recognition and production; I am fine with answers that are specialized to this task. 
It is important to explain how the neurobiological changes produce a change in learning that seems so qualitatively different and sudden compared to the gradual decreases in learning at other times.




Related questions

How is a young child able to learn language so easily?

Why does neuroplasticity decrease in adults?

Computational models of early learning in children
The truth is, we don't fully know. There are likely a number of factors. One of them seems to be the Brain Derived Neurotrophic Factor (BDNF). This molecule seems to be involved with maintaining and encouraging neuron growth. In Rats with a transgenically accelerated rise in BDNF levels, the critical period began earlier, and finished earlier as well (Huang et al., 1999). This doesn't exactly get at the heart of the question, because clearly there is some interaction between experience and expression of this gene. It's a good starting point, however.
References
Huang, Z.J., Kirkwood, A., Pizzorusso, T., Porciatti, V., Morales, B., Bear, M.F., Maffei, L., &amp; Tonegawa, S. (1999) &quot;BDNF Regulates the Maturation of Inhibition and the Critical Period of Plasticity in Mouse Visual Cortex&quot;, Cell 98(6): 739-755. doi:10.1016/S0092-8674(00)81509-3