I think that might be because that is when they generally graduate from high school and start getting out on their own in life.
Well lets take this slowly.
17 years olds are still legally minors but cross the line when they act like adults and commit MURDER!
If you are 17 you can ask the court to recognize you as an adult if you wish to be independent of your parents, so many 17 years old can be recognized as adults.
Now if a 17 year old gangsta breaked into your home and raped and killed your family would you view them as a little child?
I had the same thought. If a 17 year old isn't allowed the same priviliges as an 18 year old, then why should he be given the same punishment? It's contradictory to say that he is too immature to drink, smoke and vote, and then say that he should be held to the same level of responsibility as an adult if he commits a murder.