The risk of stroke attributable to atrial fibrillation increases from 1.5% for those ages 50 to 59 to 23.5% for those ages 80 to 89.
Wolf P, Mitchell J, Baker C, Kannel W, et al. Impact of Atrial Fibrillation on Mortality, Stroke, and Medical Costs. Arch Intern Med. 1998; 158(3): 229-34. http://archinte.ama-assn.org/cgi/content/abstract/158/3/229