One can use CRISPR on fully grown neurons, it just has to be done indirectly. The key is to add an extra step. Namely, to use CRISPR to engineer better viral vectors for BL-OG, bioluminescent optogenetics.

This isn’t as difficult as one might think since it’s already possible to efficiently use bespoke viral vectors to affect very specific populations of neurons for optogenetics research. With BL-OG one could do it without any fiber optic cables attached to people’s heads.