Answer by taking a quote from the following article:

Michael Joseph Piazza (; born September 4, 1968) is an American former professional baseball catcher who played 16 seasons in Major League Baseball (MLB), from 1992 to 2007. He played most notably for the New York Mets and Los Angeles Dodgers, while also having brief stints with the Florida Marlins, San Diego Padres, and Oakland Athletics. A 12-time All-Star and 10-time Silver Slugger Award winner at catcher, Piazza produced strong offensive numbers at his position; in his career, he recorded 427 home runs--a record 396 of which were hit as catcher--along with a .308 batting average and 1,335 runs batted in (RBIs). Piazza was drafted by the Dodgers in the 1988 MLB draft as a favor from Tommy Lasorda to Piazza's father.

Piazza was born in Norristown, Pennsylvania, grew up in Phoenixville, Pennsylvania, and attended Phoenixville Area High School. He is of Italian and Jewish (Slovak) ancestry, and is the second-oldest son of Vince and Veronica, with brothers Vince Jr., Danny, Tony, and Tommy. Mike grew up a Philadelphia Phillies fan, and admiring Hall of Fame Third baseman Mike Schmidt. Vince Piazza earned a fortune of more than $100 million in used cars and real estate, and attempted several times to purchase a MLB franchise. When the Dodgers--managed by Vince Piazza's childhood friend Tommy Lasorda, the godfather of Mike Piazza's youngest brother, Tommy--visited Philadelphia, Piazza visited the Dodger clubhouse and served as a bat boy in the dugout. Vince Piazza's own hopes of playing baseball had ended at the age of 16 when he left school to support his family. He saw that Mike Piazza had potential in the sport, and began encouraging his son to build his arm strength at the age of five. When he was 12, Piazza received personal instruction in his backyard batting cage from Ted Williams. The Hall of Famer praised his talent, advised him not to let anyone change his swing, and autographed Piazza's copy of Williams' The Science of Hitting. Vince Piazza threw hundreds of pitches nightly to his son, who shared his father's focus on baseball, clearing snow if necessary to practice his hitting and, after reaching the major leagues, practicing on Christmas Eve. He attended Phoenixville Area High School and graduated in 1986.

when did he start playing baseball?
began encouraging his son to build his arm strength at the age of five.