Trial at 16
Okay, I have a story revolving around a character I made named Austen Loche, and he has an addiction to a magical drug. He is able to steal it in small samples from the Medical Ward, but he is found out at the end of his sixth year, and goes to trial for it (it is a highly restricted drug). How would a sixth year be put on trial by the Ministry?
Since Mr Weasley was shocked that Harry was being tried by "the whole court," it seems that only delegations from the Wizengamot would be present at most trials. Only really serious crimes, like murder or use of the Unforgivable Curses, would seem to merit a full trial. Also, Mr Weasley seemed confused that it was "down in Old Courtroom Ten," so most trials probably don't take place there. (You've got courtrooms 1-9 to choose from, though!)
I would imagine that he would be tried by about five to ten of the Wizengamot, NOT in Courtroom Ten. As far as Magical Law Enforcement goes, you have pretty free rein, since Jo hasn't told much about it. He would probably be arrested by Magical Law Enforcement, sent to Azkaban (*eek-dementors*), and have to wait for trial, if it's anything like the Muggle world. I would think that wizards wouldn't have lawyers and such, though. With things like Veritaserum to force truth, that would be rather unnecessary.
Hope I helped!
schwartzandgrubennierre said a lot, but I just want to add a few things. I think that, going off Mr. Weasley's dialogue in that chapter, Courtoom ten is maybe the only courtroom where the full Wizengamot is used.
Also, we don't have a lot of information about their legal system, but according to the HP Lexicon, wizards don't have defense attorneys or prosecutors. The closest thing you would have to a defense attorney would be a "witness for the defense," as Dumbledore was to Harry in OotP. In place of the prosecution, you would have what ever presiding officer was overseeing the trial, like Cornelius Fudge, also in OotP.
Witnesses can also be called forth.
Oh, and about Veritaserum, didn't JKR say either in the books or out, that it wouldn't be used on criminals on trial and such because people have ways of blocking it?
I always found it odd that the Wizarding world does not have a system specifically designed to handle juveniles as far as justice was concerned. That they would treat a minor the same as an adult is disturbing.
But that is just my two knuts from a parental viewpoint.