What is the importance of a number on a baseball jersey? How long have they been wearing numbers?
All teams began wearing numbers on their jerseys in 1937 though some wore them inconsistently prior to that.
In 1929 the Yankees started wearing numbers on their jerseys. Their numbers corresponded to their place in the batting order. In other words Babe Ruth batted third, therefore he was #3. Lou Gehrig batted fourth so he wore #4.
Today, it is quite fashionable for young guys to emulate their favorite player by wearing his number on their little league uniforms.