I think any time a juvenile makes the conscious decision to commit a crime like, drink alcohol, smoke, sexual assault, assault, attempt to kill or kill someone, destroy property or any crime that an adult would be held responsible to the letter of the law, that juvenile should be held accountable for their actions as an adult.
A crime is still a crime no matter the age. The severity of the crime requires the investigation into the motivation, mental status, and conscious mind of the perpetrator.....not maturity.
A death penalty case would be the result of someone taking a life with malice and forethought, not because it was an accident or the defendant didn't know what they were doing.
Note: There are countries that train and enlist children (10 to 17) to kill and be killed....these are babies emulating adults that kill or die trying. Sounds far from juvenile to me.