The Bible speaks of two generations or beginnings as genesis’. That can be seen in the plural use of generation (generations ) when they, not it as if one was created
Gen 2:4 These are the generations of the heavens and of the earth when they were created, in the day that the LORD God made the earth and the heavens,
One, the first generation the generation of Adam also called the evil generation as natural un converted man describing those who walk by sight after the rudiments of this world . Its genealogy ended when the father confirmed ....Thou art my beloved Son; in thee I am well pleased. It traced back to Adam, as seeds (many) fleshly.
And the Holy Ghost descended in a bodily shape like a dove upon him, and a voice came from heaven, which said, Thou art my beloved Son; in thee I am well pleased.And Jesus himself began to be about thirty years of age, being (as was supposed) the son of Joseph, which was the son of Heli,Which was the son of Matthat, which was the son of Levi, which was the son of Melchi, which was the son of Janna, which was the son of Josep Luk 3:22
Which was the son of Enos, which was the son of Seth, which was the son of Adam, which was the son of God. Luk 3:38
Second is the generation of Christ it began at the birth of the Son of man ending the genealogy of the spiritual seed (one) Christ, which began with Abraham.
Mat 1:1 The book of the “generation of Jesus Christ”, the son of David, the son of Abraham.
Mat 1:17 So all the generations from Abraham to David are fourteen generations; and from David until the carrying away into Babylon are fourteen generations; and from the carrying away into Babylon unto Christ are fourteen generations.
It is in respect to the new creation. The first resurrection when the Old Testament saints came out of the graves.
In lieu of the new, then the Lord says, I tell you the truth, "this generation will not pass away until all of things have happened." It would be the same as saying heaven and earth shall pass away but his law that governs this creation will not pass until all things are fulfilled.
He was not promoting that we seek after signs .They were signs of observation as in things will on as they are until the last day. Just like in the time of Noah wars and rumors of wars, men marrying having families business as usual . Christ said it is an evil generation (the generation of Adam) that does seek after signs. In requiring a sign before they commit faith unto our faithful Creator .they stumble over the cross walking by sight..
There were those who had thought His kingdom had come.
Mark 13:31 Heaven and earth shall pass away: but my words shall not pass away.
Paraphrased it would be.... this generation of Heaven and earth shall pass away: but my words of my law shall not pass away.
Luke 16:17 And it is easier for heaven and earth to pass, than one tittle of the law to fail.