I think potty training has wide variability but also multiple windows, regardless of high IQ. If you miss one window, another will come along, though it might be a while. DS was showing readiness at 24 mo but I was due with DD and the idea of regression due to sibling stress kept us from acting on it. I also felt he lacked the motor skills to dress himself at that time. He learned at 3, but night took far longer (deep sleeper). DD was fully self-trained by 20 mo, including nights.