adjective
a word that describes or modifies a noun, in this case, 'adultlike' describes something that is characteristic of an adult
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The child displayed a level of maturity that was quite adultlike.
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She's so responsible and organized, very adultlike for her age.
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He's acting all adultlike now that he has a job and pays bills.
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The way she handled the difficult situation was very adultlike, showing wisdom beyond her years.