What does that have to do with the mental capacity to understand that rape and murder are wrong?
Did you understand that murder and rape, and theft were wrong at, say age 9? age 11? age 13? why or why not?
Again, these were not kids playing doctor. These were kids participating in the brutal rape of two women that ended putting them up in the hospital. If that doesn't trigger a "wait a minute.... this is wrong...i'm out of here" reaction then like Sinflix say, they don't deserve to live. Are you really saying that they did not know that they were doing something wrong so they don't deserve punishment?
This isn't about consenting to sex or understanding a contract. This is about committing a crime. Two different mindsets. Two different parts of the brain.