If you commit a fairly serious crime (like stealing a car) when you are 17 but the trial is put off until you turn 18, can you be tried as an adult even though you committed it as a minor?

Depending on the charges against you and (when the charges were brought), it is possible.  It is truly dependent upon all of the facts. To help give you an overall idea of Arizona law, this response will give a general overview of  charging minors as adults

Charging Minors as Adults 

Generally, if a minor is 15 years of age or older, and is accused of a violent crime (including, but not limited to: murder, forced sexual assault, and armed robbery) or the minor (15yrs or older) is considered a "chronic felony offender" then the county attorney/prosecutor is directed to bring adult charges against the minor; A.R.S. § 13-501(A);  (A chronic felony offender is defined as  "a juvenile who has had two prior and separate adjudications and dispositions for conduct that would constitute a historical prior felony conviction if the juvenile had been tried as an adult;" ARS § 13-501 (G)(2) )

 If a minor is 14 years or older, the county attorney/prosecutor can decide to bring adult charges against the juvenile if:

• The juvenile (14yrs or older):
  • is accused of any Class 1 or  Class 2 felony;
  • is accused of a Class 3 felony in violation of any offense in chapters 10 through 17 or chapter 19 or 23 of the criminal code; (this includes a broad range of categories);
  • is accused of a Class 3, Class 4, Class 5 or Class 6 felony where there was a serious and intentional physical injury involved; (and/or the threat of serious injury was involved); or;
  • is considered a "chronic felony offender" See, A.R.S. § 13-501 (B);