The reasons for having a juvenile system are similar, but trial as an adult isn’t based on a criminal’s perceived maturity and therefore responsibility and cognizance for their actions - it’s based on a societal need to remove someone who despite their immaturity is committing heinous crimes beyond what the juvenile system is designed to deal with. It’s not a recognition of that child as an adult, it’s simply a way to change what sentencing options are available for the judge.
