There are two ways to consider doubt as I see it. Sometimes doubt exists in a person as an obstacle to truth, a persistent doubt that causes a person to reject the most basic truth. You have experienced this I am sure. I have one child who is a very picky eater. The other day I was trying to get her to taste something new (I forget what it was now). I know what she likes and had tried it and I was convinced she would like it, but her doubt disabled her from accepting the truth of what I was telling her. This is doubt rooted in fear. That is important because the solution to this kind of doubt is not mental persuasion, but overcoming the fear.
There is another kind of doubt though that is healthy. This kind of doubt has more to do with being fully aware of our own mental and physical limitations, our own propensity to think wrong or be deceived. In my experience, teenagers lack this doubt altogether, as it comes with age and experience. It is more a healthy skepticism about our ability to know things as they are. I think this doubt is healthy because it clearly sees and understands the gap between what we know and what we believe, that is what we can reason and what we must take on faith.
People who lack this doubt do not have faith, they only have ideology. The truth is that my understanding of God is always limited and faulty, not because God is faulty, but because I have a limited finite mind that can only reach so far. Faith is a gift of the Holy Spirit precisely in that God reaches across that gap. Kant is right in that I cannot reach up to God, but Kant is wrong in that God does reach down to me. And the only people who can appreciate this fact are those who have enough healthy doubt to feel the limit of their reach and the gap that leaves.