The term ‘Generations’ in the Bible is used to refer to an era, covering to the duration of the life of a prophet or the live/reign of a monarch. The Generation of Abraham; the generation of Moses; the generation of David etc; ending with ‘the generation of Christ’, the current generation. Which has begun when Jesus was born and will end after this world has been terminated through his judgement.
REV 21: 1 And I saw a new heaven and a new earth: for the first heaven and the first earth were passed away; and there was no more sea.
Therefore you must not be confused when you read Jesus quote:
MAT 24: 34 Verily I say unto you, This generation shall not pass, till all these things be fulfilled.
MAR 13: 30 Verily I say unto you, that this generation shall not pass, till all these things be done.
LUK 21: 32 Verily I say unto you, This generation shall not pass away, till all be fulfilled.
This is the generation of Jesus, and it does not stretch a mere 20-25 years, but several thousand years.