Again, you have not presented a case for actions nattering more than age, with age being representative of mental capacity.
But let's pretend you have,
"These findings confirm that adolescents generally have a greater tendency towards impulsivity, making unsound judgments or reasoning, and are less aware of the consequences of their actions.
Because of their immaturity, adolescent children are also more likely to be coerced by adults and are sometimes the pawns for more sophisticated criminals. They are also more likely to be taken advantage of during the investigation of a criminal case. Juveniles are often intimidated by adults and authority figures, and are therefore more likely to be the victims of coerced confessions, which are often false. Moreover juveniles are less likely to invoke their Miranda Rights, including their right to legal representation. Most importantly, the goals of the death penalty do not apply to juveniles. Retribution aims to give the harshest punishment to the worst offender. Juveniles are the most likely to be capable of rehabilitation. Given their emotional immaturity and lessened culpability, they are not among the ""worst of the worst.""
https://www.aclu.org/other/juveniles-and-death-penalty
I'll remind you my point was that there needs to be a line somewhere. Not a specific point about this tragedy and the person responsible.