The biggest reason to raise the legal age to 21 is to reduce young people’s access to tobacco when they are more likely to become addicted and when their brains are still developing. Studies have found that nicotine, the main addictive ingredient in cigarettes, can impair cognition among young people. About 90 percent of adult smokers first use cigarettes before turning 19, and almost all smokers start before age 26, according to an Institute of Medicine study published last year.