"Then some of the scribes and Pharisees said to Him, "Teacher, we want to see a sign from You." But He answered them, "An evil and adulterous generation demands a sign, but no sign will be given to it except the sign of the prophet Jonah. For as Jonah was in the belly of the huge fish three days and three nights, so the Son of Man will be in the heart of the earth three days and three nights. The men of Nineveh will stand up at the judgment with this generation and condemn it, because they repented at Jonah's proclamation; and look--something greater than Jonah is here! The queen of the south will rise up at the judgment with this generation and condemn it, because she came from the ends of the earth to hear the wisdom of Solomon; and look--something greater than Solomon is here!"
(Matthew 12:38-42 HCSB)
First off, let it be stated that Jesus did in fact come in the flesh to a specific generation. It was the generation of first century second temple Judaism. Secondly, when you look at the Old Testament, and when you see the word generation it is always the result of the Hebrew word DOR/dore. The meaning of DOR/dore is always a forty-year generation. And, it should be noted that in the Old Testament, much was made of the forty year period of a generation. They wandered in the wilderness for forty years because of the disobedience of a specific generation. Further, Jesus being a Jew, raised as a Torah Observant Jew, attending the Torah school that all young Jewish boys attended, it follows that his thought and speech patterns would be akin to first century, second temple Judaism. He would have a strong knowledge of Hebrew, Hebrew thought and language, and would be very familiar with the language of the Old Testament.
Secondly, when the text refers to the scribes and the Pharisees, it is actually referring to rabbis who spent their time studying, discussing and teaching the Torah and Tanakh (Old Testament.) So then, when he addresses them, and goes on to say that they are an evil and adulterous generation, he is directing their thought processes back to Deu 1:35. In Deu 1:35 the reference is definitively to a forty-year revolution of people. It is the pronouncing of the sentence on the unbelieving who would not enter the promised land. There is such a strong connection between the Old Testament promised land of Canaan and the spiritual promised land of Sabbath Rest. The writer of Hebrews devotes chapters three and four to this premise.
Thirdly, in the above text, Jesus is replying to their request for a sign. He very frankly states that they will be given no sign other than his death, burial, and most of all his resurrection. The Ninevites repented from the preaching of Jonah and they (this generation) have a greater than Jonah in their midst. Further, the Queen of Sheba will stand in judgment of them (this generation) as she came seeking King Solomon, and a much greater King than Solomon was in their midst. One by one as we look at Jesus references to "this generation" we will see that he meant a generation of forty-years.
Now then, it becomes absurd when theologians try to jump through linguistic hoops to deny that Jesus meant that whenever he used the phrase "this generation" and most especially, in Matthew 24:34; Luke 21:32; Mark 13:30. I am convinced that whenever Jesus spoke the words translated as "this generation' he meant the forty-year revolution of time that makes a Jewish generation.