from entering the land. Only Caleb and Joshua, the two men of courage and conviction, are granted an exception. If the covenant were, then, exclusively or primarily for the living, it would be stillborn. But that does not address the question of how the consent of the original generation can bind those who come later. 
Part of the answer, as noted above, is that the covenant is conceived not just as enabling relationships with individuals but also as communal. Israel is a nonderivative party to it, both as beneficiary and giver of commitments. At the very least, this means that the covenant is not rendered null and void by the demise of the last person to have been present at Sinai. So long as the collectivity identified as Israel continues to endure, both it and its members remain bound by a network of privileges and obligations. That, though, pushes the question back: How do those who have not chosen to join with the nation, but are simply born into it, inherit duties they did not voluntarily assume? 
The nature and extent of interconnectedness in scripture between individual and communal responsibility pose difficult interpretive problems. In the space of just one chapter God declares to Moses, "I have considered this people, and I see that they are a stubborn people. Now let me alone to vent my anger upon them, so that I may put an end to them," and then, "It is the man who has sinned against me that I will blot out from my book" (Exodus 32:9-10; 33). 
Membership status in Israel is primarily opt-out rather than opt-in. If one is born to parents who are citizens of the nation, then one inherits that status. (If the parents are of mixed citizenship, then, according to most traditions of interpretation, it is the status of the mother that is determinative.) Birth, however, is not destiny. Those who enter the world lacking an Israelite identity can go through a process of naturalization (i.e., conversion) that brings them into the fold. Joining up is difficult, opting out more so. If, however, one knowingly and deliberately forsakes the old covenant for a new covenant, membership in Israel is thereby surrendered.
It would be an exaggeration to say that the process is painless and free of recrimination -but sundering loyalties is rarely frictionless. 
One problem with the normative standing of opt-out rules is that they often trade on inertia. People who find themselves enrolled in some association without having done anything whatsoever to bring about that connection, perhaps without knowing how they got there or what they would have to do in order to extricate themselves, cannot be said to have chosen their status. This is not to say that the status is unmerited - criminals may merit punishment, for example, even though they do not choose it -but whatever justification the status possesses does not derive from consent. Under an opt-out system, genuine voluntariness depends on people being afforded knowledge of the conditions of membership and the location of the exit door. They must also be able to leave without suffering drastic penalties. The paradigmatic example is conversion to Christianity. 
The ceremonial celebration of Bar Mitzvah is of relatively late origin, sometime in the Middle Ages. The female equivalent, Bat Mitzvah, is much later still. Because these rites only underscore a change in legal status that occurs independently of the ceremony, they are of no relevance to association membership and are only peripherally relevant to the communication of its significance.