I'm also not sure how far ignorance goes as an excuse although this could well depend on whether we are talking about civil or criminal law. For example in pretty much any country there are literally thousands of laws that you are expected not to break. I doubt even veteran lawyers know all of these down to the letter , yet if I am charged with one of them that I have no knowledge of I cannot get away with saying that I didn't know it existed. In theory I guess it could be argued that you should never do anything without first consulting a legal professional.
Possibly a lawyer could say to grandma "If you didn't know anything about routers or Java updates, why didn't you hire an IT expert to configure your computer for you?"