immature

/ˌɪməˈtʃʊr/
adjective
  1. Not fully grown or developed; young.
    • The doctor said the baby's lungs were still immature at birth.
    • Immature plants need extra water and protection from the sun.
    • The immature birds still have their baby feathers.
  2. Behaving in a way that is not as sensible or responsible as expected for someone of that age.
    • Stop being so immature and take responsibility for your actions.
    • It was immature of her to storm out of the meeting just because she disagreed.
    • His immature jokes annoyed the older students.
Antonyms
What does "immature" mean? | whatsthatwordmean | whatsthatwordmean