immature

Use the adjective immature to describe something that is not fully grown: “Elizabeth rescued an immature sparrow that had fallen from its nest during the storm.”

When used in a general sense, immature describes something in an early stage of development, such as an immature fruit tree or an immature political system. When applied to people, however, immature is often used to describe someone who behaves in a childish way. So if your 42-year-old brother spends the entire holiday weekend playing video games and then throws a temper tantrum when you ask him to help with the dishes, the word you’re looking for to describe him is probably immature.

Definitions of immature
  1. adjective
    not yet mature
    Synonyms:
    adolescent
    in the state of development between puberty and maturity
    embryologic, embryonal, embryonic
    of an organism prior to birth or hatching
    inchoative
    beginning to develop
    larval
    immature of its kind; especially being or characteristic of immature insects in the newly hatched wormlike feeding stage
    prepubertal, prepubescent
    (especially of human beings) at the age immediately before puberty; often marked by accelerated growth
    prepupal
    of an inactive stage in the development of some insects, between the larval and the pupal stages
    pubescent
    (of animals especially human beings) having arrived at the onset of puberty (the age at which sex glands become functional) but not yet fully mature
    pupal
    of the insects in the chrysalis (cocoon) or post larval stage
    underdeveloped
    not yet fully developed
    premature
    born after a gestation period of less than the normal time
    young
    (used of living things especially persons) in an early period of life or development or growth
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    Antonyms:
    mature
    having reached full natural growth or development
    adult, big, full-grown, fully grown, grown, grownup
    (of animals) fully developed
    abloom, efflorescent
    bursting into flower
    fruiting
    capable of bearing fruit
    full-blown, matured
    fully ripe; at the height of bloom
    headed
    of leafy vegetables; having formed into a head
    marriageable, nubile
    of girls or women who are eligible to marry
    overblown
    past the stage of full bloom
    meridian, prime
    being at the best stage of development
    old
    (used especially of persons) having lived for a relatively long time or attained a specific age
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  2. adjective
    (used of living things especially persons) in an early period of life or development or growth
    synonyms: young
    new
    not of long duration; having just (or relatively recently) come into being or been made or acquired or discovered
    one-year-old
    one year of age
    two-year-old
    two years of age
    three-year-old
    three years of age
    four-year-old
    four years of age
    five-year-old
    five years of age
    adolescent, teen, teenage, teenaged
    being of the age 13 through 19
    infantile
    being or befitting or characteristic of an infant
    boyish, boylike, schoolboyish
    befitting or characteristic of a young boy
    childlike, childly
    befitting a young child
    early
    very young
    girlish, schoolgirlish
    befitting or characteristic of a young girl
    junior
    including or intended for youthful persons
    little, small
    (of children and animals) young, immature
    newborn
    recently born
    preadolescent, preteen
    of or relating to or designed for children between the ages of 9 and 12
    puppyish, puppylike
    characteristic of a puppy
    tender
    young and immature
    youngish
    somewhat young
    vernal, young, youthful
    suggestive of youth; vigorous and fresh
    junior
    younger; lower in rank; shorter in length of tenure or service
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    Antonyms:
    old
    (used especially of persons) having lived for a relatively long time or attained a specific age
    experienced, experient
    having experience; having knowledge or skill from observation or participation
    mature
    having reached full natural growth or development
    aged, elderly, older, senior
    advanced in years; (`aged' is pronounced as two syllables)
    aged, of age
    having attained a specific age; (`aged' is pronounced as one syllable)
    ageing, aging, senescent
    growing old
    ancient
    very old
    anile
    of or like a feeble old woman
    centenarian
    being at least 100 years old
    darkened
    (of fabrics and paper) grown dark in color over time
    doddering, doddery, gaga, senile
    mentally or physically infirm with age
    emeritus
    honorably retired from assigned duties and retaining your title along with the additional title `emeritus' as in `professor emeritus'
    gray, gray-haired, gray-headed, grey, grey-haired, grey-headed, grizzly, hoar, hoary, white-haired
    showing characteristics of age, especially having grey or white hair
    middle-aged
    being roughly between 45 and 65 years old
    nonagenarian
    being from 90 to 99 years old
    octogenarian
    being from 80 to 89 years old
    oldish
    somewhat elderly
    over-the-hill, overage, overaged, superannuated
    too old to be useful
    sexagenarian
    being from 60 to 69 years old
    venerable
    impressive by reason of age
    senior
    older; higher in rank; longer in length of tenure or service
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  3. adjective
    (of birds) not yet having developed feathers
    synonyms: unfledged
    unfeathered
    not having feathers
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    Antonyms:
    fledged, mature
    (of birds) having developed feathers or plumage; often used in combination
    fledgeling, fledgling
    (of a young bird) having acquired its flight feathers
    full-fledged, fully fledged
    (of a bird) having reached full development with fully grown adult plumage; ready to fly
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  4. adjective
    not fully developed or mature; not ripe
    synonyms: green, unripe, unripened
    unaged
    not subjected to an aging process
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    Antonyms:
    mature, ripe
    fully developed or matured and ready to be eaten or used
    aged, ripened
    of wines, fruit, cheeses; having reached a desired or final condition; (`aged' pronounced as one syllable)
    mellow, mellowed
    having a full and pleasing flavor through proper aging
    overripe
    too ripe and beginning to turn soft
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  5. adjective
    characteristic of a lack of maturity
    immature behavior”
    Synonyms:
    adolescent, jejune, juvenile, puerile, sophomoric
    displaying or suggesting a lack of maturity
    babyish
    characteristic of a baby
    childish, infantile
    indicating a lack of maturity
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    Antonyms:
    mature
    characteristic of maturity
    autumnal
    characteristic of late maturity verging on decline
    mellow, mellowed
    having attained to kindliness or gentleness through age and experience
    ripe
    at the highest point of development especially in judgment or knowledge
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