The oldest active MLB player as of 2019 is Bartolo Colon. He was born on May 24, 1973, making him 45 years old.
Colon began his professional baseball career in the Dominican Summer League in 1994. He made his Major League debut with the Cleveland Indians in 1997 and also played for the Montreal Expos, Chicago White Sox, Los Angeles Angels of Anaheim, New York Yankees, Oakland Athletics, San Francisco Giants, Atlanta Braves and Texas Rangers before joining the Minnesota Twins in 2018.
He has been a four-time All-Star (1998, 2005, 2013 and 2016) and won the American League Cy Young Award in 2005 while with the Angels.