In short yes it stacks. Sage gives a set bonus of 40 exp(20 in Crystal Scar) for kills and assists. Once the total exp amount is calculated(including Sage's bonus) the bonus from Awareness and/or Zilean's Heightened Learning is added on. So potentially, with all 3 that 40exp could turn into 45.2exp!
Note: These masteries no longer exist in the current game