Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
septŭāgēnārĭus, a, um, adj. [septuageni], of or belonging to the number seventy, containing seventy, septuagenarian: fistula, seventy quarter-digits broad, Front. Aquaed. 56: homo, a man of seventy, a septuagenarian, Dig. 50, 6, 5, § 7.