Eighteen a. : Eight and ten; as, eighteen pounds.

Eighteen n. : The number greater by a unit than seventeen; eighteen units or objects.

Eighteen n. : A symbol denoting eighteen units, as 18 or xviii.

Example:
How can four Eighteen year olds get a hotel room?

Taxonomy
United States, car number, free encyclopedia, Canadian provinces, number 18, voting age, parental consent, 18A, syllable, equals, confusion, science, square, History, proper divisors, aliquot sum, circles, armies.

Defination from the web:
Eighteen is the second abundant number with the aliquot sum of 21 (117% in abundance) in the aliquot sequence (18,21,11,1,0) thus being the first ... Eighteen is also the age in many countries at which a person may appear in a pornographic video and in some countries express legal consent to ...