Definitions of immature:

• adjective:   not yet mature
• adjective:   characteristic of a lack of maturity

  Example: "Immature behavior"

• adjective:   lacking in development

  Example: "Immature plans"

• adjective:   (used of living things especially persons) in an early period of life or development or growth
• adjective:   (of birds) not yet having developed feathers
• adjective:   not fully developed or mature; not ripe