Doug Mason commented: "The anonymous people who wrote that Gospel and put the word "this generation" on to Jesus' lips were meaning their own generation, not some people living at some arbitary time in the future.
They got it wrong, just as Paul got it wrong (1 Thess), as did the writer of Revelation. They all thought the Coming was imminent, during their own time, the time of their generation."
This hints at the simplest way to understand it. Let's say that Jesus actually did say "this generation". The simplest way to understand it is that Jesus was referring to the people he was talking to and who lived at that time. And the destruction of Jersusalem in 70 A.D. fits that "generation" by any measure of the definition, since it occured only 37 years afterward.
Any other attempt to apply to some future time is problematic, which the folly of the WTS's speculations has born out.