The juvenile-to-adult (J/A) transition, or puberty, is a period of extensive changes of animal body morphology and function. The onset of puberty is genetically controlled, and thelet-7miRNA temporally regulates J/A transition events in nematodes and mammals. Here, we uncover the targets and downstream pathways through whichCaenorhabditis elegans let-7controls male and female sexual organ morphogenesis and skin progenitor cell fates. We find thatlet-7directs all three processes by silencing a single target, the post-transcriptional regulatorlin-41. In turn, the RNA-binding protein LIN41/TRIM71 regulates these processes by silencing only four target mRNAs. Thus, by silencing LIN41,let-7activates LIN-29a and MAB-10 (an early growth response-type transcription factor and its NAB1/2-orthologous cofactor, respectively) to terminate progenitor cell self-renewal and to promote vulval integrity. By contrast,let-7promotes development of the male sexual organ by up-regulating DMD-3 and MAB-3, two Doublesex/MAB-3 domain–containing transcription factors. Our results provide mechanistic insight into how a linear chain of post-transcriptional regulators diverges in the control of a small set of transcriptional regulators to achieve a coordinated J/A transition.