bright young thing

/B R AY1 T Y AH0 NG TH IH0 NG/
noun
  1. 1

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

  2. 2

    Any of a group of bohemian young aristocrats and socialites in 1920s London.

Source: https://en.wiktionary.org/wiki/bright%20young%20thing