eighteen

Etymology

From Middle English eightetene, eiȝtene, ehtetene, from Old English eahtatīene, eahtatēne, from Proto-Germanic *ahtōutehun. Cognate with West Frisian achttjin, Dutch achttien, German achtzehn, Danish atten. Equivalent to eight + -teen

num

1. The cardinal number occurring after seventeen and before nineteen, represented in Roman numerals as XVIII and in Arabic numerals as 18.
   The right of citizens of the United States, who are eighteen years of age or older, to vote shall not be denied or abridged by the United States or by any State on account of age. 1971, Twenty-sixth Amendment to the United States Constitution