Sure. When you commit first degree murder fully knowing that what you are doing is wrong.
There is your line.
Oh well then...you are in favor of making it even MORE arbitrary.
Most 7 year olds understand that murder is wrong. But with cartoons and video games...maybe they don't understand that shooting someone might actually KILL them. Same can be said for even some 14 year olds. Who will make the decision then...A psychologist, a judge, YOU?
We have made the age of majority 18. That is the law in the land. We simply must accept that children are viewed differently by LAW.