"Substance abuse" or "alcohol and other drug abuse" (AODA) means a pattern of use of alcohol or other drugs with the potential of leading to immediate functional problems or to alcoholism or other drug dependency, or to the use of alcohol and/or other drugs solely for the purposes of intoxication. The term also means the use of illegal drugs by persons of any age, and the use of alcohol by persons under the age of 21. [20 ILCS 301/1-10] (PG 99.13)