Is ovarian cancer an unusual occurance in a 20 year old girl?

Quite rare: From 2005-2009, the median age at diagnosis for cancer of the ovary was 63 years of age. Approximately 1.3% were diagnosed under age 20; 3.6% between 20 and 34; 7.4% between 35 and 44; 18.6% between 45 and 54; 23.4% between 55 and 64; 20.1% between 65 and 74; 17.6% between 75 and 84; and 8.1% 85+ years of age. See: http://seer.Cancer.Gov/statfacts/html/ovary.Html.