RE: Should the Eighth Amendments apply to an underage defendant who committed a capital punishment?

Ms. Carolina Gomez, makes a good argument stating that she feels the eighth amendment should apply to everyone, including people underage. Her reasoning behind her statement is that it could potentially decrease the violence that is increasing in the youth. Carolina's problem is that children who create capital offenses should receive the punishment that has been put forth for the crime. But instead as she references the Thompson vs. Oklahoma Supreme Court case, the judges in this said that offenders under the age of 18 can not be sentenced to death due to "cruel and unusual punishment". So why if it is cruel for someone who is underage, why isn't it cruel for someone older? I agree with Carolina's argument, she made her statement and she had evidence, if you commit the crime you do the time, in this case you loose your life!