Final answer:
Christopher Simmons was sentenced to life in prison instead of the death penalty because he was not yet 18 years old at the time of the crime. This was determined in the Supreme Court case of Roper v. Simmons in 2005.
Step-by-step explanation:
The fact that Christopher Simmons was not yet 18 years of age led to his being sentenced to life in prison instead of the death penalty. In the case of Roper v. Simmons in 2005, the Supreme Court ruled that it is unconstitutional to impose the death penalty on juvenile offenders. The decision concluded that it is cruel and unusual punishment to sentence individuals who were under 18 at the time of the crime to death.