Refer this article in Journal of Video Experiments for better insight into isolation protocol.

http://www.jove.com/video/562/preparation-of-dissociated-mouse-cortical-neuron-cultures.

Also, if you need an in vitro surrogate, N1E-115 cell lines is a mouse neuroblastoma and are commercially available.

Collaborators of ours have a protocol for culturing from P1 mice which is very handy if you are using transgeneic mice and don't want to kill the dams to protect your stock levels. Taking the cortex at E14 is far more usual though and gives a higher yield.