act one's age

act one's age idiom
Behave more maturely. Although the phrase often is used in asking children to act in a more grown-up fashion (Only babies suck their thumbs; act your age), it also may refer to an adult who is, sometimes deliberately, acting much younger than might be considered appropriate (Grandpa, it's time you stopped climbing ladders and acted your age).

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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