Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
septŭennis (collat. form septennis, Capitol. M. Aurel. 21), e, adj. [septem-annus], of seven years, seven years old: nunc priusquam septuennist (puer), Plaut. Bacch. 3, 3, 36; so, puer, id. Men. prol. 24; 5, 9, 57; id. Merc. 2, 2, 21; id. Poen. prol. 66; Prud. στεφ. 10, 656.