By "trying as an adult" the suggestion is already that the person is guilty, which is against law and custom in all but the most retrograde legal systems.
No, "Trying as an adult" does not suggest that the person is already guilty, or you could move straight to sentencing. When someone is sent to trial there must be some evidence which points to that person being guilty, but it is then put before Magistrates, or a Jury to decide whether the evidence is sufficient.
In UK at least, there is a presumption of innocence, which continues until such time as a guilty verdict has been achieved.