Traditionally, used to denote the beginning of (American)
baseball season practice anxiously awaited by enthusiasts each springtime: If there was
grass growing, then the season had progressed enou gh past winter to start the new season: "play ball!" More recently, it has become a slang inferring statutory laws regarding underage females did not, in practice, realistically apply and that instead, the presence of pubic hair (
grass) indicated sufficient maturation to begin sexual activity (play ball).
"I know your
sister's only twelve, but I go by
baseball, not age, "If there's grass on the field, play
ball!"