noun
a person who is fully grown or developed
adjective
having reached full size and strength; mature
preposition
in the room
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During the meeting, it was clear that he was the most mature adult in the room.
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When it comes to decision-making, she's definitely the grown-up in the room.
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He's the real adult in the room when it comes to handling conflicts.
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In a room full of chaos, she always manages to be the grown-up and keep things under control.