Well, it depends on age.
An 18 year old is considered an adult by the legal system. Is this enough life experience though for one to truly make "adult" decisions?
Sometimes it is and sometimes it is not.
There have been cases I believe, where someone would be spared the death penalty for an exceptionally low IQ, it would be said they have a "mind of a child," and so could not be given an adult punishment, but it is still right and necessary to permanantly remove that person from society.
However, western society in generally, more so for the U.S. it seems than other countries, and on some level it may simply be a human condition, that people just don't want to grow up.
At some point, a person of sound mind has to have the capacity to realize that what happened in their childhood does not have to effect their actions.
They need to grow up, and I see it all the time, not so much murders, but on the streets, grown men in suit and tie throwing fits like children because of traffic. Adults crying over the fast food restaraunt not getting their order fast enough, people calling 911 for the silliest things. There was a grown man in Florida who had a python who escaped its cage once. There was a 2 year old kid in the house, it escaped again and killed the kid.
These adults are acting like children and it needs to stop. Whether it's the atmosphere we live in where no one is at fault, the idea that parents need to be friends with their children instead of authority figures, that we expect government to put warnings on everything to sensor everything to tell us what we can or cannot do, whether these things are to blame, or whether it's just who we are I don't know. But I think it's important we do something different in our society so more of its people know how to be adults.
So no, a person's childhood should not be any kind of consideration in any kind of criminal proceding.