Who was the youngest of the twelve Olympian gods?

Answer 1

Hera was youngest, Demeter the middle daughter, and Hestia the eldest, but they came out in order of most recent birth when Zeus disgorged them from Cronus, so Hera would then be "elder", Demeter still middle, and Hestia "youngest".

Answer 2

Either Hermes or Dionysus. Chronos was the youngest of the Titans, the generation before the Olympians.