When an act, which would otherwise be a certain offence, is not that offence, by reason of the youth, the want of maturity of understanding, the unsoundness of mind or the intoxication of the person doing that act, or by reason of any misconception on the part of that person, every person has the same right of private defence against that act which he would have if the act were that offence.
You have the same right of private defence against someone who cannot be held criminally liable (child, insane, intoxicated) as you would if they could be held liable.