Why is adolescence a critical time for preventing drug addiction?

Science of Addiction- National Institute of Drug Abuse  (beginning of information)

Early use of drugs increases a person’s chances of developing addiction. Remember, drugs change brains—and this can lead to addiction and other serious problems. So, preventing early use of drugs or alcohol may go a long way in reducing these risks. If we can prevent young people from experimenting with drugs, we can prevent drug addiction.