Jesus was plainly referring the generation to whom he was speaking at that time. This may well be in addition to a future generation that should fulfill, in type, the same conditions, but it most certainly applied to the Jews then living.
How do we know?
Jesus was only seconding a prophecy that had already been given through Ezekiel. Ezekiel had prophesied both destructions of the temple in Jerusalem, giving timelines of 390 years from the rebellion of Jeroboam until the first destruction, and 40 years from the "iniquity of Judah" to the second one. Those prophecies are found in the fourth chapter of Ezekiel, with the times being particularly in focus in verses 4-6.
4Lie thou also upon thy left side, and lay the iniquity of
the house of Israel upon it: according to the number of the days that
thou shalt lie upon it thou shalt bear their iniquity. 5For
I have laid upon thee the years of their iniquity, according to the
number of the days, three hundred and ninety days: so shalt thou bear
the iniquity of the house of Israel. 6And when thou hast
accomplished them, lie again on thy right side, and thou shalt bear
the iniquity of the house of Judah forty days: I have appointed thee
each day for a year. (Ezekiel 4:4-6, KJV)
So Ezekiel had been commanded to lay on his right side 40 days for Judah's iniquity, with each of those days to represent a year.
The sin of Judah is written with a pen of iron, and with the point of
a diamond: it is graven upon the table of their heart, and upon the
horns of your altars; (Jeremiah 17:1, KJV)
That expression "pen of iron" invokes thoughts of nails by which Jesus would be crucified. And sure enough, at the end of one generation, defined as 40 years in the Bible (see Numbers 14:34), after Jesus was put to death by the Jews (Judah), the prophecy is fulfilled. The second temple, with all of Jerusalem, is destroyed by the soldiers of Titus.
Many among those Jews who heard Jesus speak, and rejected him, would be among those upon whom the prophecy was shockingly fulfilled. That was "THIS" generation, literally.
Because the word applies to that literal generation, however, it must not be assumed that no other generation would be exemplified in parallel, as a secondary or dual fulfillment of the prophecy. The prophecy is highly likely to have a second application to the end of time, just before Jesus' return.