No matter how severe the crime, I don't believe in trying juveniles as adults.
Or I would under the following circumstance: If the juvenile is found "not guilty", said juvenile has the right to vote, drink, drive, seek employment, enlist in the military, rent a car, rent a hotel room, sign a contract, engage in sex, gamble, etc.
I'm not one to be fond of laws, any laws, but when a law says "the government must behave in this manner" I'm strict about it. The government has decreed that 18 shall be the age of adulthood, and the age of adulthood shall be 18. It's not an absolute decree, as one can drive at 16, drink at 21, and various states have various ages of consent. But overall, 18 shall be the age of adulthood, and the age of adulthood shall be 18.
Trying a juvenile as an adult says "you have none of the rights of being an adult but you do have the responsibilities of being an adult." Also it says "we made this rule, but we can break it when we feel like it."