Your argument, then, also precludes prison.
No, it does. Kids under 18 have some rights, just not the same rights as adults. Similarly, they must be judged according to laws and the consequences of their actions, but not the same laws as adults. A murder is a horrible crime when committed by a 16 year old as by a 25 year old. But age is a mitigating factor, the same as premeditation and mental state. That's why the judicial system recognizes several degrees of murder and manslaughter--every situation has different rules.