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

adj 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
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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adj (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
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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adj (of birds) not yet having developed feathers

Synonyms:
unfledged
unfeathered
not having feathers
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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adj not fully developed or mature; not ripe

Synonyms:
green, unripe, unripened
unaged
not subjected to an aging process
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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adj characteristic of a lack of maturity

immature behavior”
Synonyms:
adolescent, jejune, juvenile, puerile
displaying or suggesting a lack of maturity
babyish
characteristic of a baby
childish, infantile
indicating a lack of maturity
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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