I don't think it's actually true that someone with:

1. A desire to learn calculus 2. A good teacher 3. No mental impairments such as dementia or other major brain drainers

could not learn calculus. Most people don't really care to try or don't get good resources. What you see as an intelligent mathematician is almost always someone born with better resources that was also encouraged to pursue math.

1 and 3 are loopholes large enough to drive a semi truck through. You could calculate how far the truck traveled if you have its acceleration with a double integral, however.