This is really an issue for us. On one hand, there is a great deal of statistical data that yields a correlation between performance on our “skills assessment exams” (plus the math part of the ACT/SAT exam) and success in our freshman calculus courses.
On the other hand, students from underserved educational districts in our state tend to do poorly on said exams and are thereby excluded from starting with calculus.
The reality appears to be that our university really can’t make up for the poor start that many students get prior to arriving here.
And yes, aptitude matters. Aptitude is a factor of genetics and the environment in which the student was raised (having great genes for math aptitude could well be overridden by poor nutrition, a polluted environment and a lack of exposure to proper grade school education).
And there is some research which suggests that if some ideas are not learned by a certain stage of childhood, they are unlikely to ever be learned.
No answers here; just a lot of frustration with trying to be as fair as possible to everyone while giving the prepared students the course that they deserve.