Our Parsha, Devarim, that is read always before Tisha Beav, contains Moshe's rebuke of Am yisrael for the sin of the spies. "Five things happened to our forefathers on Tisha BeAv", the first in the list isn’t the destruction of the temple, it’s the decree that our forefathers will have to stay in the desert for forty years, and not enter the land of Israel. Moshe addresses the young generation, and analyses the sin. He speaks mainly about one thing "Emuna" – faith. This was the test, and we failed it. This is the beginning of our sorrows on Tisha BeAv, and the strengthening of our faith is the key to Tikkun, to mending our ways.
The audience hearing Moshe does not include anyone aged between 60 and 100, as they all died in the desert, apart from Yehushua and Calev. The men listening and receiving the rebuke were either born in the desert or were children at the time of the sin, together with people aged over a hundred, who were aged over sixty at the time.