Of course, according to traditional Catholic dogma, men are born into a state of condemnation even before they commit any mortal sins; that's called, "original sin."
Yes. No man is promised, owed or can earn salvation, which is a gift from God.
And to say God foresees the mortal sins that some infants might commit if they were permitted to grow to adulthood and die in mortal sin, and decides to end the lives of those infants early, is a speculation
No, it's not a speculation. Many saints have said it's a fact.that just proves my main point: God chooses those infants as opposed to others
If by "chooses" you mean a type of predestination, then yes, there something called 'catholic predestination'. But...this does not mean that God is "not fair" to the damned. It simply means he is "more generous" in grace to those whom He knows will accept such. As in the gospel of man who goes out and hires the idle laborers to work in the field. Christ saves some men in the twilight of their lives, while others come into the Faith from birth.
This is God's plan, but we can't say it's "unfair" to those laborers who rejected the offer to come work in the field. The gospel does not mention how many laborers turned down the offer to work, but surely there were some. And they were not saved.
who he lets age and commit mortal sins and go to hell.
But you're denying actual grace here. Everyone who commits a mortal sin was given the grace not to. As St Paul tells us, infallibly, that God will not permit us to be tempted beyond our strength. This is the doctrine of actual grace, which ALL MEN receive, every second of their life, whether catholic, protestant, jew, etc. If anyone commits a mortal sin, it is their choice. If they go to hell, it is ultimately their choice.Thus, He purely gratuitously favors one sinner who deserves hell over another whom He consigns to it.
God does not consign anyone to hell. This is heresy.
You are denying free will; you are denying actual grace; you are denying God's salvific will. You need a reality check.