peace ay, all,
Note: AQ = al quran
I have not studied AQ with regard to this topic specifically, but from my studies so far, here is my view:
According to AQ, a person becomes fully responsible from adolescence onwards and when they have rushd*. This is when they can get married, can be given their inheritance etc, ergo, equal in all respects to an adult as far as I am aware.
*be well guided or directed, true direction, correct rule of action, straight forwardness, maturity of a child/intellect, capacity to manage one's affairs.
If the person does not fulfil this criteria, I don't see how they can be held responsible in the same way. It would perhaps then default to a community decision, which could range from: forgiveness/nothing, detainment / house arrest, compensation, or even once they fulfil the criteria they could opt to receive the punishment. If God exists, then there is no doubt that expiation in this life is much better than requital in the next one.