The first document issued by the US federal government authorizing a person to fly an aircraft was issued on April 6, 1927 by the Aeronautics Branch of the Department of Commerce, and it is called a license, not a certificate. When did the US stop issuing pilot "licenses" and start using the word "certificate"?
757toga said that his father's pilot "certificate" was issued September 29, 1946, and Amelia Earhart's "license" was issued in 1930, so the change almost certainly happened at some point between 1930 and 1946. (Earhart's license was last endorsed in 1935, but this doesn't prove that a new authorization issued between 1930 and 1935 wouldn't have been called a certificate.)