Mine didn't start until I hit puberty. I experience everything other than the after-effects of the expanding-contracting, though the nightmares seriously impact my concentration for a long time afterwards. I don't get any visuals to fuel the fear, but I just get inexplicably scared. But the math is something that really rings true to me -- I often feel extreme amounts of anxiety because I feel as if I must express my thoughts or feelings through mathematical equations that I am otherwise unaware of the function of. I've also had dreams in which I could only speak in Spanish, of which I'm only partially fluent. It's just really frightening & I'm overcome with anxiety.

EDIT: My IQ is also in the upper range, about 147 when I was last tested, & I have schizoaffective disorder/pure O OCD. Not sure if any of that is relevant but I figured I'd share it since you made sure to offer background details.

Also, my boyfriend used to have nightmares about being chased around by numbers & being swallowed up by them. I'm not sure if that is related or not but it seems to be.