The wise of heart heed commandments.
Jesus gave a new commandment: To love one another as He loved people. He gave Himself for His people.
If we are wise, we will do as He said and deny ourselves, giving our lives for others in love.
The length of generations varied but was often represented in the OT by 40 years (in the Dead Sea Scrolls, 40 years represents the suffering of the final generation). Jesus speaks these words near A.D. 30; the temple would be destroyed in A.D. 70.
Jesus is right about everything.
Even if you don’t like it.
Even if it does not fit what you were always taught or what you always thought.
He just is.