Between the ages of 40 and 49; in one's fifth decade(adjective)
One who is quadragenarian(noun)
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Use "quadragenarian" in a sentence
"He, a stalwart, well-oiled quadragenarian, shone with sweat from his exertions, brandishing cooked pigs."
"Two hours later, what had been the erect image of a gigantic coal – porter turned miraculously white, was now no more than a medley of disjected members; the quadragenarian torso prone against the pedestal; the lascivious countenance leering down the kitchen stair; the legs, the arms, the hands, and even the fingers, scattered broadcast on the lobby floor."
"Musil, who describes an incestuous passion between brother and sister, and Lolita, by Vladimir Nabokov, which describes the passion of a quadragenarian for"