What are possible consequences for violating probation with a under age possession of alcohol charge under NY criminal laws

It depends on the terms of their probation: if there was an agreement not to use alcohol (usually there is) then there may be some additional jail time involved and an extension of probation equal to the period of time you were in violation. This is not the crime of the century.

You will probably be charged with a probation violation if the terms of probation included a "no alcohol" clause. The charge will result in a probation violation hearing being scheduled, likely before the original sentencing judge. You are entitled to be represented by counsel and your attorney should try to work out a settlement with your probation officerr, prior to the hearing. If it does go all the way to a hearing the P.O. will act as the prosecutor (the ADA for that court will be there, but generally they turn over the proceedings to the P.O.). In order to be found to have violated your probation the judge must find that there was a preponderance of evidence supporting that finding (basically, that means more evidence supporting the finding than not). Your attorney can cross-examine the P.O. and can present witnesses on your behalf (you have a right to testify if you wish to).

You have two issues here. First you were put on probation for the original charge of stealing. That offense has a range of potential penalties that start with probation and go up to jail time depending on the exact charge, your prior record, etc. You were given a light disposition on this and placed on probation. One term of probation is always that you obey all laws. Since you committed a new crime, you violated your probation. The original charge now comes back into play and it is as if you have two open charges, the original stealing and the new underage possession of alcohol. Assuming that these are the only two entries on your criminal record, the court will probably just add an extra drug and/or alcohol class or another month or two to your probation and give you a slap on the wrist for the underage possession charge.

There is no 6 month probation in NY State so what you are referring to is probably an Adjournment in Contemplation of Dismissal (ACD) An ACD means your case is adjourned for 6 months. As long as you don't get arrested during those 6 months the case goes away as if it never happened. That is a common result for a shoplifting case. If you then got re-arrested during those 6 months, then the old shoplifting case could get restored to the calendar and a different plea offer would come. Underage possession of alcohol in a car is not a crime, but a violation. It is possible that no one will know about the ACD if you just go in and pay the fine on the alcohol offense.