While the Age-60 rule presumes that pilots become less safe with advancing age, recent research suggests that youth comes with dangers of its own. The part of the brain that inhibits risky behavior does not mature until age 25, according to a study by the National Institutes of Health. "We'd thought the highest levels of physical and brain maturity were reached by age 18, maybe earlier -- so this threw us," researcher Jay Giedd told The Washington Post. The research has been cited by lawmakers in efforts to restrict the driving privileges of young people. Driving accidents are the greatest cause of accidental death in ages 16 to 24. The research shows the age of "brain maturity" varies widely, and tends to be reached earlier by women. Giedd's study analyzed brain scans of 2,000 people between the ages of 4 and 26. Another study showed that young people tend to take greater risks when their friends are watching, an effect that persists until about age 24.