In the MBTI system, we don't say that the inferior function does'nt work but that it works submit to the dominant function. And that the tertiary function is submit to the auxillary. With SPs, Ni is submit to Sebut ork though. And mny SPs, young or not are actually big Ni users, even if iy's often quite unconcious. We just say that at the "middle-age" (abstract notion who does'nt really mean anything), the inferior function begin independent, and so, an ESTP can use Ni like an INFJ or INTJ, an ENFP can use Si like an ISTJ or an ISFJ, an ISFP can use Te like an ENTJ or an ERSTJ. The tertiaary function become independant at the "young adult age", theorically, though.