Hensch, T. .
Brain function is enduringly shaped by early life experiences, when neural circuits exhibit windows of robust plasticity then later stabilize. A biological basis for these "critical periods" has begun to emerge: the local balance of circuit excitation-inhibition drives onset timing, while molecular ?brakes? actively limit rewiring thereafter. Manipulations targeting these processes are so powerful that individuals of identical chronological age may be at the peak, prior to, or past their plastic window. Thus, critical period timing per se is plastic. Strikingly, most of these regulators converge in and around pivotal inhibitory circuits which are particularly vulnerable in mental illness. Understanding their maturation and maintenance offers novel therapeutic insight into cognitive disorders and the potential to tap juvenile levels of plasticity throughout life - for better or worse.