It is written (Job 1:14): "The oxen were ploughing, and the asses feeding beside them," because, as Gregory expounds this passage (Moral. ii, 17), the simple, who are signified by the asses, ought, in matters of faith, to stay by the learned, who are denoted by the oxen. [ST II-II, Q2, A6]
This indicates that not merely that diminished capacity (as in the case of infants or the mentally handicapped) but even education is in view as something that affects one's obligation for explicit faith. But even thorough education doesn't imply an absolute duty for believing explicitly in literally every dogma of the Church, because the Faith is not a merely rational thing. It is supernatural, and exceeds the capacity of reason.
The unfolding of matters of faith is the result of Divine revelation: for matters of faith surpass natural reason. [ibid.]
People have varying measures of intellectual and educational capacity, but none of us is able to fully grasp every jot and tittle of the Faith. Still, those with greater gifts ought to be able to explicitly believe more than the rest of us, and consequently more is expected of them. And this, of course, is what the Bible teaches us.
And that servant, who knew the will of his lord and prepared not himself and did not according to his will, shall be beaten with many stripes. 48 But he that knew not and did things worthy of stripes shall be beaten with few stripes. And unto whomsoever much is given, of him much shall be required: and to whom they have committed much, of him they will demand the more. [Luke 12:47-48; emphasis added]
In the words of Ben Parker: "With great power, comes great responsibility."
This doesn't mean that those of less capacity are so off the hook that they can believe just anything: implicit faith doesn't imply no obligation whatsoever. We must still do the best we can, and our intent and purpose to believe the truth is surely part of what it means to have implicit faith – for those of us who are able to form such intentions. The grace that saturates the Gospel taught by the Catholic Church is surely evident in this, that God does not demand more of us than we can actually do (and even then it is his grace that enables us to do it).