Well Jesus said it would happen BEFORE the next generation.. 40 years. The Scofield Heresy has become very popular with the Evangelicals and fundamentalists.. Then Hal Lindsey made a fortune promoting bad theology.
There is no where that Jesus said before the next generation.
What Jesus did say
in the book of Mark 13:30--
" Verily I say unto you, that this generation shall not pass, till all these things be done"
Jesus also said in Matthew 24:34
"34 Verily I say unto you, This generation shall not pass, till all these things be fulfilled"
You do know, Jesus is pointing ahead to the future, to the generation that will witness the beginning and end of the Tribulation which we are now heading into.
For some unknown reason, people listen to much to man's teachings and not the teachings of Christ Jesus.
As to how you come with the next generation is anyone's guess
As you can see there is no where in Mark 13:30, and Matthew 24:34 that Jesus made any mentioning of the next generation.
Jesus is pointing ahead to the future of the generation that will witness everything that Jesus given in Mark 13 and Matthew 24 and Luke 21.
All these things will happen which is still future from us and that generation will witness all those things that Jesus given in Prophecy
Mark 13, Matthew 24 and Luke 21.