Neuroplasticity is the brain's capacity to reorganize itself by forming new neural connections and adjusting existing ones, in response to learning, experience, injury, and environmental change. Plasticity is highest in childhood (a widely cited 2000 study by Maguire and colleagues found that London taxi drivers, who spend years memorizing the city's complex street layout to pass "The Knowledge" licensing exam, show measurably enlarged posterior hippocampi compared to controls — evidence of adult structural plasticity from sustained learning, not just a childhood phenomenon), but it continues throughout adult life, just at a somewhat reduced and more effortful pace.
The practical significance is that 'you can't teach an old dog new tricks' is neurologically false as a hard limit — it's true that new learning generally requires more repetition and sustained effort in adulthood than in childhood, but the underlying mechanism (strengthening and forming new synaptic connections through repeated use) remains available across the entire lifespan, which is the basis for adult skill acquisition, rehabilitation after brain injury, and cognitive training more broadly.