When Jesus said, “this generation” will see the events of the Olivet Discourse prophecy fulfilled, he didn’t mean the literal generation of his contemporaries alive in the first century. He meant “this current evil age” in which most people are opposed to the Kingdom of God.
In essence, Jesus was warning his disciples that wickedness will prevail on the earth until his Second Coming, while also encouraging them that in due time, God will set all things right at the time of his parousia. Here is a link to a great article that explains the meaning of the phrase “this generation” (Matt. 24:34) in the Olivet Discourse.