<p>Recent advances have shifted our understanding of the blood–brain barrier (BBB) not as a static wall, but as a dynamic and evolving system whose property is variable across the lifespan: acquisitive in early development, heterogeneous in adulthood, and increasingly fragile in aging. This BBB dynamic has called for a conceptional shift in how we understand the physiological and pathological processes related to barrier functions, including CNS drug exposure and age-related neurological disorders (e.g., stroke, vascular dementia). This Editorial offers a lifespan-based perspective of the BBB dynamics, highlighting current knowledge on how the BBB evolves from development through adulthood and into aging. Overall, recognizing the developmental and dynamic nature of the BBB is essential for safer pediatric dosing, more predictable adult therapeutics, and effective vascular strategies for preserving cognitive health.</p>