“Rabbi Shimon Ben Elazar says, ‘If children tell you ‘Build!’ and elders tell you ‘Break!’ heed the elders and do not heed the children, for children’s building is breaking, and elder’s breaking is building, and the sign of the matter is Rehoboam, son of Solomon’” (Nedarim 40a).
RASHI interprets that if youths tell you, “Build the Temple,” and elders tell you, “Break and do not build,” the sign of the matter is Rehoboam… and the Temple was ruined over that advice. But the Torah did not interpret about the Temple, and there is a Braita that interprets “Temple” differently, and see MAHARSHA.
We should interpret that the elders say that we must break all the building of Kedusha [holiness] that a person built in his youth, for otherwise it is impossible to build the Temple. The reason is that one must revoke everything that he has done since he was born so he can walk in the ways of Kedusha [holiness].