Everyone here knows that Joe Nuxhall was the youngest major league player ever, pitching for the Reds in 1944 at age 15. Nuxhall pitched 2/3 of an inning, allowing five runs on 2 hits and 5 walks.
The oldest Red to play was right handed pitcher Jack Quinn, who was signed as a free agent by the Reds on May 6, 1933, one week after being released by the Brooklyn Dodgers. Quinn appeared in 14 games, pitching 15 2/3 innings, allowing 20 hits and walking 5. He was 0-1 with a 4.02 ERA. His last appearance came on July 7, two days after this 49th birthday.
(Information gathered from “Day by Day in Cincinnati Reds History” by Floyd Conner and John Snyder, and baseball-reference.com.)