When brain scans were analyzed, activation was seen in regions of the brain of the youngest pot smokers that was not seen in the smokers who started after age 16. Researchers at Harvard-affiliated McLean Hospital found that this indicates early exposure to marijuana can cause neural changes. Early and habitual users of marijuana did not do well on tests of cognitive functions such as mental flexibility.

Study author Staci Gruber, the director of the cognitive and clinical neuroimaging core at McLean Hospital in Belmont, Mass. claims that research on how marijuana changes a developing brain is very important, because it is the most frequently used illegal drug in the United States. A 2009 study from the National Institute on Drug Abuse found that around 16 percent of eighth graders have tried marijuana, and the amount continues to increase to about 42 percent by 12th grade.

The most extreme rise in use was seen in the 55- to 59- year- olds, whose marijuana use more than tripled from 1.6 percent in 2002 to 5.1 percent. It is expected that there will be more increases in use as the 78 million boomers who were born between 1945 and 1964 get older. Many boomers tried it decades ago. Some have been smoking it ever since and others are trying it again now that they are in retirement, either for fun or as a way to help with the aches and pains of getting older. One lady interviewed said that marijuana has helped her get better sleep than using pills ever did. Another man who suffered from degenerative disc disease and arthritis said that he was amazed at how well it worked after previously turning to all kinds of drugs for his crippling pain. He now claims that he is virtually pain free.

All this said, Dr. William Dale, chief of geriatrics and palliative medicine at the University of Chicago Medical Center, warns that older people who use marijuana are at possible risk for falling if they get dizzy. He also said that smoking increases the risk of heart disease and it can lead to cognitive impairment.