 Within nations , there is a slightly higher prevalence in higher socioeconomic groups . Males and females are affected in an equal ratio , and the peak age of onset between 10 and 19 years . About 80 % of people with aphthous stomatitis first developed the condition before the age of 30 . There have been reports of ethnic variation . For example , in the United States , aphthous stomatitis may be three times more common in white @-@ skinned people than black @-@ skinned people . 
