  Quantum mechanics together with general relativity leads to the
  K\'arolyh\'azy relation and a corresponding energy density of quantum
fluctuations of space-time. Based on the energy density we propose a dark
energy model, in which the age of the universe is introduced as the length
measure. This dark energy is consistent with astronomical data if the unique
numerical parameter in the dark energy model is taken to be a number of order
one. The dark energy behaves like a cosmological constant at early time and
drives the universe to an eternally accelerated expansion with power-law form
at late time. In addition, we point out a subtlety in this kind of dark energy
model.
