Every kid is different.
For, my DD10 the curve has flattened she still makes impressive leaps at times and the gradient is still steeper than most kids even years older than her.
The curve is more of an arc with some bumps (steps) in it from what I have observed in my DD.
I think that the curve apparently flattens as they learn more and more about the world and develop broader and more varied interests. Sort of like the way that a river meanders as it reaches its floodplain.
But, as others have succinctly put it, when you have seen one gifted kid you have seen one gifted kid...
