I do think that some juveniles who commit felonies should be tried as adults but not always. Think about a 7 year old who stole from the post office. Technically they committed a felony, but do we really need to try them as an adult? No. But if a serious felony is committed then it should be considered.
Juveniles who commit felonies should be tried as adults because of the severity of their crimes. The perpetrators who committed these crimes acted as adults. When they chose to commit these crimes, they had no problem being adults. When they stand before the judge for sentencing, they should also be prepared to be treated as adults.
There are times when juveniles should be tried as adults. This should be reviewed on a case by case basis. Things to consider include age, history, motive, and severity of the crime. If they are high enough on enough of those factors they should be tried as an adult in our legal system.
Felonies are more serious crimes and juveniles should be tried as adults if they are old enough. Teens who beat people up, pull a gun on someone or rob a store already know that behavior is wrong and should be punished accordingly. For a child to be accused of a felony is a fairly serious matter that needs serious consequences.
With many offenses the juvenile should not be tried as an adult. However, if it is a heinous crime that was premeditated and the young person is at least 16 years old, then perhaps that would be necessary. In most cases, there should be some attempt at rehabilitation for a teen.