When can you start attacking cities in Rise of Kingdoms?

Probably from any level.... But you don't really want to do that, you'll lose a lot of troops, best to wait a long time before you make any city attacks. If you want to damage someone else's City, best to go for their farmers when they are out of the City. But pay attention to your alliance and kingdom rules as attacking other Cities or farmers may only be allowed at certain times, for example during the kill event in the Mightiest Governor.