IDIOMS

bright young thing

(idiomatic, sometimes capitalized) One who is youthful, clever, eager, and high-spirited in manner and attractive in appearance.

Example: 1869, R. D. Blackmore, Lorna Doone: A Romance of Exmoor, ch. 10:
  Mr. Faggus gave his mare a wink, and she walked demurely after him, a bright young thing, flowing over with life, yet dropping her soul to a higher one, and led by love to anything; as the manner is of females.
1902, Bret Harte, "Golly and the Christian" in Condensed Novels Second Series: New Burlesques:
  And even as her pure young voice arose above the screams of the departure whistle, she threw a double back-somersault on the quarterdeck, cleverly alighting on the spikes of the wheel before the delighted captain.
  "Jingle my electric bells," be said, looking at the bright young thing, "but you're a regular minx—"
1918, Stephen Leacock, "The New Education" in Frenzied Fiction:
  "So you're going back to college in a fortnight," I said to the Bright Young Thing on the veranda of the summer hotel. "Aren't you sorry?"
  "In a way I am," she said, "but in another sense I'm glad to go back. One can't loaf all the time." . . .
  How full of purpose these modern students are, I thought to myself.
1952 Jan. 14, "The Press: Strictly Personal," Time (retrieved 22 Aug 2014):
  YOUNG COLLEGE MAN, travelled, slightly peeved and irked, not disenchanted, would relish hearing from bright young things with gay outlook, brilliant notions.
1994 March 20, Nigel Cope, "Bunhill: Above the crowd without a net," The Independent (UK) (retrieved 22 Aug 2014):
  Charles Wigoder, the 34-year-old chief executive of Peoples Phone, the mobile telephone business, is very much a bright young thing—the kind of businessman who features in magazine articles called '40 under 40', alongside other rising stars who have done unlikely things at unusual ages.
2007 Dec. 3, Janet Maslin, "Dear Alfred, Gertie and Mummy-snooks: Love, Noelie" (book review of The Letters of Noël Coward ), New York Times (retrieved 22 Aug 2014):
  As a bright young thing—which, it could be argued, is what he remained until almost his dying day—Noël Coward wrote letters filled with effusive glee.