Maturity and circumstance should always be taken into the account.
The law takes intent more seriously than anything else, which is why you have the large distinction between the punishment for murder versus manslaughter - death sentence versus life imprisonment is quite a huge leap if you ask me.
There is definitely no set age for when one becomes mature emotionally or intellectually, that's for sure - there are old people who still behave like kids, and not because they are aged, but because of their mentality and view of life. There are 12 year olds who have literally seen so much in life.
I do believe more in second chances than anything else though. You can say - hey, but what if he does it again? Then he didn't deserve that second chance, but I think everyone does have some amount of right to make mistakes along the way.
Reform programs and counseling for minor offenders - even young murderers are much more successful than those who are more aged. To me, this just proves that minors should be given more leeway than the older guys.