Yes, there's no reason students shouldn't be taking calculus by the age of 15(being conservative, which highlights America's lackluster schools).

In any case, my experience with some of these textbooks is that they use only one way of teaching the material when mathematics has many many formulas that can achieve the same desired outcome. I suppose the thought is that the classroom doesn't have enough time to go over the many possible techniques or am I wrong? If so why do so many textbooks have only one example for achieving an answer when there are probably more than 4 on every equation?