Answer by taking a quote from the following article:

Mays was born in 1931 in Westfield, Alabama. His father, Cat Mays, was a talented baseball player with the Negro team for the local iron plant. His mother, Annie Satterwhite, was a gifted basketball and track star in high school. His parents never married.

Mays began his major league career on a sour note, with no hits in his first 12 at bats. On his 13th at-bat, however, he hit a towering home run over the left field roof of the Polo Grounds off of future Hall of Famer Warren Spahn. Spahn later joked, "I'll never forgive myself. We might have gotten rid of Willie forever if I'd only struck him out." Mays's batting average improved steadily throughout the rest of the season. Although his .274 average, 68 RBI and 20 homers (in 121 games) were among the lowest of his career, he still won the 1951 Rookie of the Year Award. During the Giants' comeback in August and September 1951 to tie the Dodgers in the pennant race, Mays' fielding and strong throwing arm were instrumental to several important Giants victories. Mays was in the on-deck circle when Bobby Thomson hit the Shot Heard 'Round the World against Ralph Branca and the Brooklyn Dodgers to win the three-game playoff 2-1 after the teams were tied at the end of the regular season.  The Giants went on to meet the New York Yankees in the 1951 World Series. In Game 1, Mays, Hank Thompson and Hall of Famer Monte Irvin comprised the first all-African-American outfield in major league history four years after the color line was broken. Mays hit poorly while the Giants lost the series 4-2. The six-game set was the only time that Mays and retiring Yankee ace Joe DiMaggio would compete against each other.  Mays was a popular figure in Harlem. Magazine photographers were fond of chronicling his participation in local stickball games with kids. It was said that in the urban game of hitting a rubber ball with an adapted broomstick handle, Mays could hit a shot that measured "six sewers" (the distance of six consecutive New York City manhole covers, nearly 300 feet).

When did he start playing baseball?
1951