The Pittsburgh Pirates signed Welch in November 2011. He began the season with the Bradenton Marauders. In his first game with the Marauders, which was against St. Lucie, Welch was hit by a Ryan Fraser pitch with the bases loaded in the bottom of the 11th inning, to score Gift Ngoepe and give the Marauders a walk-off win. Welch's first promotion to Double-A came in June 2012, when he was promoted to the Double-A Altoona Curve. In November 2012, Welch re-signed with the Pirates.
Stefan attended Norwood Morialta High School and graduated in 2006  He also played cricket, football, and basketball for local Adelaide clubs and school teams. He still currently plays Division 1 Baseball for the Goodwood Indians  His mother, Karen Young, lives in South Australia and is the Accounts Manager at Bridgewater Constructions. His sister, Brooke Young, born in 1998, also lives in South Australia and studies at St. Peters Girls College  His fiancée and high school sweetheart Shannon Lodge currently lives in South Australia working as an events planner and professional dancer. She also travels with Welch to the United States supporting him and his profession. His father Darren Welch is currently living in Washington D.C. and is the Manager at Edge Concrete. He also played in the Olympics for Australia