noun
BAD : Sitting next to me was a middle age man, probably about forty.
GOOD : Sitting next to me was a middle-aged man, probably about forty.
BAD : The Rolling Stones are all middle-age now.
GOOD : The Rolling Stones are all middle-aged now.
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The adjective is middle-aged (WITH -d ): 'They would prefer a middle-aged woman to a young girl.'