"Testing everyone in this age range removes stigma associated with the test, thus reducing barriers for the provider and the patient," the group added.
The Centers for Disease Control (CDC) recommended last year that every baby boomer get tested for hepatitis C, which many people carry without knowing it.
The virus often has no symptoms at the time of infection. It may be transmitted by sharing needles, or through a bad blood transfusion or organ transplant.
Dr. John Ward, head of the CDC's viral hepatitis division, said many older people who injected drugs in their youth have unknowingly carried hepatitis C for decades.
"We had an epidemic of hepatitis C transmission in the '70s and '80s, and we're now seeing an epidemic of hepatitis C disease," he told NPR last August.
The National Viral Hepatitis Roundtable is seeking additional signatures to its letter before delivering it to Sebelius on May 1 for Hepatitis Awareness Month.