A. The short answer is "the person's age." A person who is 21 years of age or older cannot be convicted of a DUI, only a DWI. An element of the crime of DUI is that the person was under 21 at the time of the offense.
A person who is be charged with either a DUI or a DWI. Typically, if a person under 21 is alleged to have had a BAC result of .08 or higher, then the officer will charge him or her with a DWI. The penalties for a DWI are generally much more severe than those of a DUI.