Around 1-1/2 - 2 years, according to my mother. That was when she noticed things like how I would spend hours fascinated with the patterns on the linoleum and I didn't play with toys the same way other kids did. The meltdowns and stim behaviors started shortly after that, I guess. I don't have any memories of being like an NT kid, and my first memories start in toddlerhood, so I guess that supports it too.