Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sexennis, e, adj. [sex-annus], of six years, six years old: erus, Plaut. Poen. 4, 2, 80; 5, 2, 27: cervi, Plin. 8, 32, 50, § 116: sexenni die, after six years, an interval of six years, Caes. B. C. 3, 20.
sexennĭum, ii, n. [sexennis], a period of six years, six years: puer subripitur Sexennio prius quam moritur pater, Plaut. Poen. prol. 67: tribuni plebis tulerunt de provinciis contra acta Caesaris, ille biennium, hi sexennium, Cic. Phil. 5, 3, 7; so id. Div. 1, 44, 100; id. Att. 6, 1, 5.