In society, we look at an 18 year old as an adult and they are held accountable for their decisions and actions. Sometimes, youth younger than 18 are 'tried' as adults. Any consequences for criminal actions are placed upon them.
But society has often felt that we need to make exceptions. In cases where it is perfectly clear on ‘who’ did it and ‘what’ was done, the legal platform no longer tries to convince anyone that the defendant is not guilty of the action… rather they pursuit an approach that the person should not be held accountable for that action for a variety of reasons: Mental issues, environment, society, family situation, etc… and that the 'accountability' needs to be shifted away from the defendant.
When parent(s) has/have done all they could to teach their children the laws of the land, how to function in society and how to properly treat other people and supplemented the teaching and educating by their own example they are not accountable for the actions of their child beyond the age of adulthood. The teaching also needs to be accompanied with some level of corrective actions when poor decisions are made.
However, when a parent(s) does/do not provide the appropriate teaching, example nor corrective actions to inappropriate behavior they are allowing a child to develop habits that will result in negative consequences. Accountability should be shared.