Neuronal critical periods are limited phases of lifetime during which the plasticity of neural connections is at its maximum and the development of the brain is adapted to the environment. Researchers at the Scuola Normale Superiore in Pisa and the Leibniz Institute on Aging (FLI) in Jena have discovered the role of a small microRNA (miR-29) in these learning-dependent phases of plasticity. A premature increase in miR-29 concentration in young mice blocks cortical plasticity, whereas blocking miR-29 in adult animals induces plasticity typical for younger sensitive phases; an indication that miR-29 is an age-dependent regulator of developmental plasticity.