“Each year that the onset of substance use is delayed until the mid-20s - about the time when the human brain is more fully developed10 — the risk of developing a substance use disorder is reduced.11 Among people who used any of these substances before age 18, one in four have a substance disorder, compared with one in 25 who started to smoke, drink or use other drugs at age 21 or later.”12

Source

Adolescent Substance Use: America’s #1 Public Health Problem," The National Center on Addiction and Substance Abuse at Columbia University (New York, NY: National Center on Addiction and Substance Abuse at Columbia University, June 2011), p. 39.
http://www.casacolumbia.org/u…