And a 30 second search returned this:
http://jama.jamanetwork.com/Mobile/article.aspx?articleid=412317
“Risk of head injury in helmeted vs unhelmeted cyclists adjusted for age and motor vehicle involvement indicate a protective effect of 69% to 74% for helmets for 3 different categories of head injury: any head injury (odds ratio [OR], 0.31; 95% confidence interval [CI], 0.26-0.37), brain injury (OR, 0.35; 95% CI, 0.25-0.48), or severe brain injury (OR, 0.26; 95% CI, 0.14-0,48). Adjusted ORs for each of 4 age groups (<6 y, 6-12 y, 13-19 y, and ?20 years) indicate similar levels of helmet protection by age (OR range, 0.27-0.40). “
Journal of the American Medical Association. 1996