90% of individuals that develop the disease of addiction do so before the age of 18 years old. As children grow up, their brain is engaged in a critical period of development. Exposing the brain to substances before the age of 25 when this brain development is complete may "rewire" the brain to crave and seek the substances that produce dopamine, the pleasure chemicals in the brain.
Children who are at elevated risk have a family history of addiction, experience trauma, disruption or change that they can't manage, or have the inability to regulate their emotions.
Genetics play a role
A study looked at 231 people who were diagnosed with drug or alcohol addiction, comparing them to 61 people who were not addicted. Looking at the first-degree relatives (parents, siblings, or children) the study discovered that if a parent has a drug or alcohol addiction, the child had an 8 times greater chance of developing an addiction.
Poor environment & coping skills.
If an individual has a genetic risk of addiction, the pathway to developing the disease becomes more probably with poor coping skills, an inability to self-regulate, and poor response to stress. Living in an environment that produces adverse childhood experiences are the other factors that elevate the risk of addiction.
An adolescent and young adult brain is not fully developed
The rational part of the adolescent brain isn’t fully developed and won’t be until age 25 or so. Repeated substance abuse triggers the brain to release dopamine at a high rate and begins to rewire the brain and arrest its normal development triggering it to seek more of the substance.