The Galactic Habitable Zone and the Age Distribution of Complex Life in   the Milky Way
  We modeled the evolution of the Milky Way to trace the distribution in space and time of four prerequisites for complex life: the presence of a host star, enough heavy elements to form terrestrial planets, sufficient time for biological evolution and an environment free of life-extinguishing supernovae. We identified the Galactic habitable zone (GHZ) as an annular region between 7 and 9 kiloparsecs from the Galactic center that widens with time and is composed of stars that formed between 8 and 4 billion years ago. This GHZ yields an age distribution for the complex life that may inhabit our Galaxy. We found that 75% of the stars in the GHZ are older than the Sun. 
Charles H. Lineweaver (1 & 2), Yeshe Fenner (3), and Brad K. Gibson   (3) ((1) UNSW, (2) Macquarie University, (3) Swinburne...