Andrew,
You asked why I dropped Jn.9:21 in the conversation. The reason is, because it is evidence that even in a culture with infant circumcision as the sign of the covenant of grace, there was still a time and place where a son passed into the realm of personal responsibility.
We may reasonably infer that the man born blind was actually still living with the parents who brought him into the world, since begging isn't likely to lead to robust self-sufficiency. The summoning of his parents by the Pharisees makes this practically certain. In other words, the situation presents itself as one in which an older child appears to be continuing "under covenant authority," inasmuch as his parents still (of necessity) provide at least a roof for him, and probably food. But they are not entirely agreeable to the officials' proposition that they bear the same relation to his adult claims, as they did when they had him circumcised at 8 days.
There is talmudic evidence that proselytes to Judaism considered the age of a parent's children when incorporating the individual and family into Jewish identity. So, it seems the questions are perennial. Family/household dynamics compel reconsideration in each situation, within the bounds of basic principle.
I agree, what should happen is not always ideally realized. Sometimes the missed "ought to" is a parental failure, as the case of Moses (Ex.4) seems to indicate; or a religious failure. Sometime the "ought to" doesn't happen because of some natural or man-made obstacle which, despite the will of the godly, prevents perfect fulfillment. And sometimes what ought to happen (when everything is in alignment) doesn't take place, because in the judgment of those with the God-given duty to make an evaluation there is sufficient reason to question the propriety of a given course of action. God will review that decision.
Should ALL people who are bona fide household participants be accorded the privilege of membership in the head-of-household's religious commitment? Yes, but not over their stiff-necked, up front objection to such incorporation. And not, I say, merely as a matter of forcing people to "go along" with the boss' dictates, and without concern for the temporal and eternal well-being of one's charges.