>>11391202The age was 4-6 IIRC, and they were hand selected for very, very specific traits, including traits that would make them more likely to be receptive to the augmentations and survive. Even with dozens of planets worth of children over the course of several years to choose from, of the 150 kids in the program, only roughly 75 survived "successfully" (at least in old lore).
This leads one to believe that, in-universe, even if these augmentations were quite doable, finding the right subject to apply them to successfully was quite difficult.
