This generation may also mean race, either Jewish or Christian. All prophecies, once fulfilled, the evil Jews, of whom Jesus is pertaining to, will then pass away because there will be a new heaven and a new earth. Evil generation or race will be completely done away with. In Luke, it was the Christian Jesus was referring to since Jesus only answered questions about the temple while in Matthew and Mark, it was all evil Jews since the end of the age was included in the questions his disciples asked. So we should not use this verse to use as proof or base text for Jesus' coming 70 to 80 years since Israel's rebirth in 1948.