As Chris Sullivan points out, the Catechism states:
God predestines no one to go to hell; for this, a willful turning away from God (a mortal sin) is necessary, and persistence in it until the end.
I'm not sure what to make of this, in particular of the parenthetical, because that's not what I understood to be Catholic doctrine. As it stands, it makes salvation the "default" destiny of mankind, and that doesn't seem to quite square with the doctrine of original sin.
In fact, the Council of Florence taught, in Session 6:
But the souls of those who depart this life in actual mortal sin, or in original sin alone, go down straightaway to hell to be punished, but with unequal pains.
The emphasized phrase is missing from the Catechism's statement. What's even more puzzling is that the first statement apparently isn't consistent with this third statement, which is also in the Catechism:
Indeed, the great mercy of God who desires that all men should be saved, and Jesus' tenderness toward children which caused him to say: "Let the children come to me, do not hinder them," allow us to hope that there is a way of salvation for children who have died without Baptism.
If, in fact, a mortal sin as usually understood is necessary to go to hell, then (since mortal sin requires the use of reason) we wouldn't merely hope there is a way of salvation for children who have died without Baptism, we would know there is no way they aren't saved. (Clearly, the children in question are understood to have not yet reached the age of reason.)
Having copied the three statements into this post, I was surprised to realize the statement about hope for unbaptized children is actually more consistent with the rather dire claim made by the medieval council. The way of salvation for which we are allowed to hope would simply need to be one that includes forgiveness of original sin. ("Simply" in the sense that it's simple to make the statements consistent, not in the sense that it's simple to work out the theology much beyond "God is not bound by the sacraments.")
I think, though, that nowadays a lot of people don't take original sin quite seriously enough, treating it as more a matter of therapy (e.g., references to our "brokenness") than of the death of the soul.