Adam Clarke on Mark 13:30

This generation - Ἡ γενεα αὑτη, This very race of men. It is certain that this word has two meanings in the Scriptures; that given in the text, and that above. Generation signifies a period of a certain number of years, sometimes more, sometimes less. In Deu 1:35; Deu 2:14, Moses uses the word to point out a term of thirty-eight years, which was precisely the number in the present case; for Jerusalem was destroyed about thirty-eight years after our Lord delivered this prediction. But as there are other events in this chapter, which certainly look beyond the destruction of Jerusalem, and which were to take place before the Jews should cease to be a distinct people, I should therefore prefer the translation given above. See on Mat 24:34 (note).