It seems like you're essentially implying the mean value theorem here. Because there exist some people who cannot do algebra (folks with mental disabilities), and there exist some people who can, there likely must be some sort of transition point where someone doesn't have mental disabilities but can't do algebra. In that graph, you're implying that "mental intelligence" is the independent variable(s) and algebraic ability is the dependent variable.

Another assumption is that the function is continuous. That's one I'm not sure about. I don't think there's a continuum between folks with mental disabilities and folks without. I think there's a discontinuity -- or perhaps overlapping spectrums on different dimensions -- but not a continuum. That's why many mental disabilities are the result of very concrete genetic variations. You can't have 80% of a generic variation. You either have it or you don't, which implies some sort of inherent discontinuity.

My guess would be algebraic ability is a normal distribution. But that's not a death sentence. People who might, because of genetics or more likely nurture/circumstance be unlikely to learn a lot in algebra, are not doomed to that fate. It just means it might require a lot more time and hard work for them and their teachers -- to the point where they might not find it feasible, and instead choose to do something else.