What neurobiological changes are associated with the end of the critical period 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