Addiction can happen at any age, but it usually starts when a person is young. If your teen continues to use drugs despite harmful consequences, he or she may be addicted.
If an adolescent starts behaving differently for no apparent reason—such as acting withdrawn, frequently tired or depressed, or hostile—it could be a sign he or she is developing a drug-related problem.
This is an excellent article put out by the US government, and it has some short videos which are very useful. Check it out HERE.