This paper: http://arxiv.org/abs/1509.02832, The Age Distribution of Potential Intelligent Life in the Milky Way, estimates the age distribution of intelligent civilazations in the Milky Way. Unsurprisingly we rank somewhat below the advancement level of termites compared to your typical ET. It is noted " By comparing the ages of these systems we estimated that 77% of potentially habitable star systems are on average 3.13 billion years older than the Sun." A civilization commensurably advanced to our own on such time scales should be easily capable of colonizing the galaxy. So, where are they? The authors note "it could be concluded that they [ET] most likely do not want their presence to be known to us yet, possibly for reasons beyond our current level of understanding."