Short answers, pulled from the story.
The Yellow Kid in McFadden's Flats appeared on the newsstands of 1897. This hardcover book reprinted material from Richard F. Outcault's newspaper comic strip Hogan's Alley which ran from the 18th of October 1896 to the 10th of January 1897.
Action Comics #1 hit newsstands in June 1938 with Superman as its cover feature. National Allied editor Vin Sullivan pulled the Siegel/Shuster creation from the slush pile to use as a backup story.
The rapid decline followed introduction of the Comics Code Authority after Senate hearings on juvenile delinquency. These hearings blamed comic books solely for social problems caused by wars of 1939, 45 and 1950, 52.
DC started a revival in superhero comics with October 1956's Showcase #4 featuring The Flash. Many historians peg this as beginning the Silver Age although Marvel had revived old superheroes as early as 1954.
Legal issues and paper shortages led decline in underground comix output from its 1972 peak. Passage of anti-paraphernalia laws in US in 1974 caused closing most head shops throttling underground comix distribution.