Adolescent idiopathic scoliosis (AIS) represents one of the most extensively studied spinal deformities, yet it continues to pose complex clinical, ethical, and methodological challenges across the lifespan. Traditionally defined as a three-dimensional spinal curvature of unknown etiology arising during adolescence, AIS has been investigated through decades of natural history studies, clinical trials, and long-term observational cohorts. The objectives of this research article are threefold: first, to synthesize and critically elaborate on the natural history and progression of AIS from adolescence into adulthood; second, to examine long-term health, functional, and quality-of-life outcomes in untreated, surgically treated, and conservatively managed individuals; and third, to explore the implications of emerging patient-centered outcome frameworks, particularly the concept of the smallest worthwhile effect, for evaluating treatment success in idiopathic scoliosis. Drawing strictly from landmark longitudinal studies, contemporary clinical reviews, and recent conceptual advances in outcome measurement, this article employs a narrative, theory-driven methodology. The findings suggest that while curve magnitude progression and radiographic outcomes remain central to clinical decision-making, they incompletely capture the lived experience of individuals with AIS. Long-term studies demonstrate that many patients with untreated scoliosis maintain acceptable function and health-related quality of life, though subsets experience pain, disability, or psychosocial burden. Surgical intervention, while effective in curve correction, introduces its own spectrum of long-term considerations, including residual pain and functional trade-offs. Integrating patient-centered metrics such as the smallest worthwhile effect offers a more nuanced framework for balancing benefits and harms across diverse treatment pathways. This article concludes that future AIS research and clinical practice must reconcile biomechanical objectives with patient-defined value, advancing toward a lifespan-oriented, individualized model of care.