Is it possible to knock out a gene in adults?
Actually, Cas9 system might be a possible method to achieve your aim in the future, research teams are trying to utilize cas9 in vivo, which means maybe someday we can doing some genetic modification in body cell precisely. However, this is only a future direction and we still can not say its fit for a knockout.(Patrick D. Hsu, Eric S. Lander, and Feng Zhang, (2014) Development and Applications of CRISPR-Cas9 for Genome Engineering. Cell 14(6): 1262-1278)
And for editing some cell gene in a limited area, some research are on going for therapeutic purpose (Liqun Zhang et al.CFTR Delivery to 25% of Surface Epithelial Cells Restores Normal Rates of Mucus Transport to Human Cystic Fibrosis Airway Epithelium), but haven't seen research are trying to built KO cell in vivo.