As a kid, you have a distorted view of reality and your prefrontal cortex is not fully developed. That is to say your morals, planning, and judgement is not fully developed. This was part of SCOTUS [outlawing capital punishment for minors](http://www.apa.org/monitor/jun05/jn.aspx).

The Prefrontal cortex is key to what we'd consider a person's personality. The seminal case of this would be [Phineas Gage](https://en.wikipedia.org/wiki/Phineas_Gage), who had his prefrontal cortex (along with most of his frontal lobe) destroyed. Despite a hole in his head, he miraculously survived. Without his prefrontal cortex, he was no longer inhibited by judgement, shame, or morals. He didn't ignore it, he just simply didn't feel it. A man went from being honest and upstanding to constantly thieving and telling lies.

Since the pre frontal cortex does not fully mature until around the age of 25, I would argue the 13 years this child does, in fact, not know the full extent of what he did.