ATLANTA — In a visit to Atlanta in 1951, new college graduate Andrew Young slowed his car when he saw a rat crossing Ponce de Leon Avenue late one night. Born in the Deep South’s New Orleans and educated in Washington, D.C., at Howard University, Young was extra cautious. About six years earlier at a conference at the Butler Street YMCA he watched the Ku Klux Klan march down Auburn Avenue — the heart of Black Atlanta. “I figured the rats had more rights than Black folks in Georgia,” he said. That may be one of the few times that Young, who turns 90 on Saturday, has slowed down in Atlanta. He m…