If you were a judge what would you do in this situation (details inside)

First of all let me explain the origin of this crazy question: last night I was brainstorming my story when an idea popped up. At first I thought it would be a great idea, but then I began to consider the possibility of its happening in real life. So I need your help to check whether it would be an appropriate idea for my story or not.

Here’s the scenario: it’s USA, 1951. There used to be an unsolved murder case in 1929 (let’s call the murdered man James). One morning there is a trial for a case seemingly unrelated to the unsolved case. We have a witness for that case called John, 30 years old. The trial soon proves a connection between the current case and the unsolved one. At that point John suddenly admits he was the true killer of the unsolved case, saying he “couldn’t go on hiding his guilt any longer” and gives some very decisive evidence. It turns out that John killed James by accident. But back in 1929 John was only 8 years old and James was an adult. So if you were the judge of that trial what would you do, give him a punishment like an adult or spare him?