The use of adjectives as nouns is common in Latin, from which some of our vocabulary and forms derive. 

>*Ave Imperator, morituri te salutant* (*Hail Emperor, those who are about to die salute you*)

*Morituri* is actually a participle form, used as an adjective, and could be translated literally *the about to be dying*.

Using adjectives as nouns in English has been common for some time.

>*Only the good die young*

(No, Billy Joel did not coin the phrase. [Wordsworth][1] said *O Sir! the good die first,
And they whose hearts are dry as summer dust
Burn to the socket.* This has morphed over time to the simpler phrase.)

This is more often found with either a definite or indefinite article.

>*The evil that men do lives after them;
The good is oft interred with their bones.*

*Marc Antony* in Shakespeare's *[Julius Caesar][2]*


  [1]: http://genius.com/William-wordsworth-the-excursion-book-i-the-ruined-cottage-annotated
  [2]: http://nfs.sparknotes.com/juliuscaesar/page_132.html