Frontiers explains what sensitive periods are: “developmental windows where maturation and specific experience interact to produce differential long-term effects on the brain and behavior”. Frontiers admits that this concept is “still controversial” but “evidence for the existence of sensitive periods has grown”.
Examples of sensitive periods are: “Children who receive music lessons, or learn a second language before age 7-8 are more proficient as adults. Early exposure to drugs or trauma makes people more likely to become addicted or depressed later life. Rat pups exposed to specific frequencies from 9-13 days post-partum show expanded cortical representations of these frequencies. Young birds must hear and copy their native song within 1-2 months of birth or they may never learn it at all.”