Because if you are charged as an adult you can be sentenced to serve longer than the age of 18
I think you were right about the age. From the State Legislature pages:
13-501. Persons under eighteen years of age; felony charging; definitions
A. The county attorney shall bring a criminal prosecution against a juvenile in the same manner as an adult if the juvenile is fifteen, sixteen or seventeen years of age and is accused of any of the following offenses:
1. First degree murder in violation of section 13-1105.