Immature means "not perfected," "not developed," "not ripe," "not complete": "All infants are immature." "Thinking that the world owes you a living is an immature concept." Premature means "too soon," "not yet ready": "The society editor made a premature announcement of the wedding." Some newborn babies are premature; all newborn babies are immature.
Dictionary of problem words and expressions. Harry Shaw. 1975.