Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† ăgērăton, i, n., = ἀγήρατον (not growing old), a plant that does not readily wither, perhaps Achillea Ageraton, Linn.; Plin. 27, 4, 4, § 13.—Ăgērătos, i, m., a designation of one of the Æons of Valentinus, Tert. adv. Val. 8.