Marijuana smoking often begins in the early teen years. This is particularly unfortunate as the young brain is particularly prone to damage.

Dr. Staci A. Gruber confirmed this at the 2010 annual session of the Society for Neuroscience in San Diego, California. She talked about a small but important study of 35 chronic marijuana smokers who averaged 22 years of age. 

Twenty of the subjects started smoking marijuana regularly before the age of 16. The rest delayed regular smoking until age 16 or later. 

All test subjects were evaluated for the quality of their executive functions. (Executive brain functions include the ability to perform abstract thinking, and planning for the future.) Executive functions also are responsible for our ability to understand rules of conduct and society. Executive functions also inhibit us from doing inappropriate and potentially harmful actions.

This study showed that teens who began smoking marijuana before age 16 had significantly impaired executive functions compared with those who started smoking at an older age. 

Those who started smoking earlier made more mistakes during the testing process and even continued to give incorrect answers even after being told that they were making mistakes. The early marijuana smokers lacked cognitive flexibility and were unable to stay focused. They were unable to follow the rules of the testing process and were less likely to control impulsive responses.

Dr. Gruber explained that at age 15 and younger the brain is still developing, and “the part that modulates executive function is the last part to develop.”

The Bible describes those who have damaged executive functions. Marijuana use by children and early teens results in the behaviors described in scripture:
  
But they refused to pay attention; stubbornly they turned their backs and stopped up their ears. They made their hearts as hard as flint and would not listen to the law or to the words that the LORD Almighty had sent by his Spirit through the earlier prophets. Zechariah 7:11-12 (NIV)

The wisdom of the prudent is to give thought to their ways, but the folly of fools is deception. … A simple man believes anything, but a prudent man gives thought to his steps. Proverbs 14:8,15 (NIV)

Ellen White, a 19th century health reformer, recognized the importance of well developed executive function when she wrote:

Thoughtful consideration is essential. If men would only think more, and act less impulsively, they would meet with much greater success in their labors. We are handling subjects of infinite importance, and we cannot afford to weave into our work our own defects of character. We want to represent the character of Christ. (Testimonies to the Church, Volume 5, p 593)