How does one use the adjectival noun "l'enfant terrible"?

Edited to add this prequel — the most common use of the phrase does not include the French article. See the statistics from Google ngram, for example:

First, the New Oxford American Dictionary definition:

enfant terrible • noun (plural: enfants terribles)
a person whose unconventional or controversial behavior or ideas shock, embarrass, or annoy others.

From the Corpus of Contemporary American English, it has quite a few hits. These include works of fiction, magazines, and some academic writings; I don't believe it is restricted to the academic style. Examples include:

• This neglect comes from the " destructive cartoon " that greeted Ellis in the early years: his image as a frivolous enfant terrible whose clipped prose was fit only for the MTV generation
• His unflattering reputation is a hangover from the Young Tom Doak era when he was architecture's enfant terrible, better known for his Golf Magazine writings on the craft than for his practice of it.
• Krenek (1900-1991) was an enfant terrible among composers, but a series of artistic setbacks prompted him to reconsider his career. In 1925, he began work on…
• Eugeni Berzin, 25, the Russian enfant terrible who won the' 94 Giro and is scheduled to make his debut in the Tour
• It also reveals that, at 71, this seminal artist still is the enfant terrible he was when he began to rattle art's cages half a century ago.
• Make no mistake, the high points of this 96-year-old enfant terrible are remarkable.

To sum it up, I would say that most uses fall under two categories: (ⅰ) young professionals or artists who are seen as behaving unconventionally or in a shocking manner; (ⅱ) in an ironic fashion, for rather older people who behave in a way that is more expected of the young.

I found only one hit for the plural form which does not refer to Cocteau’s movie:

Yet he writes with a riotous, slangy precision reminiscent of a line of British enfants terribles, beginning with Evelyn Waugh in the 1920s-30s and continuing in such contemporary figures as Martin Amis and Will Self.

You would use this expression to qualify one individual or entity who would behave badly with respect to the standards of its community.

In the past, when in families you had a child who distinguished himself by being more unruly than the others, behaving badly, breaking things or being more aggressive with his parents or brothers and sisters, then he would deserve the title of "Enfant Terrible". Bear in mind that in the Victorian era, in aristocratic families one would commonly use French to communicate thereby avoiding being understood by the servants.

In more modern times you can find the expression used in domains where one way to reach celebrity is to challenge established standards: fashion, design, art, show business.

Here are for example a few examples in the world of fashion:

• Jean-Paul Gaultier: l'Enfant Terrible of French Fashion

• Alexander McQueen: Enfant terrible and fashion genius

As you can see from the above articles, the sense of unruliness is mitigated by a suggested connotation of hidden brilliance, as if the episodic eccentricities where the natural consequence of a repressed genius.