Back in the 1970s, a now-forgotten writer claimed that 75 percent of the people who had ever been born were alive at that moment. This 'factoid' has been referenced many times, even though a bit of reflection would show how unlikely it is.
So, what might be a better estimate of the actual percentage? In this video, a Population Reference Bureau scholar explains how to get a reasonable guesstimate.
Here is an infographic look at the data.