With the advent of survival motor neuron (SMN)-enhancing therapies, the natural course of spinal muscular atrophy (SMA) has been reshaped, unveiling new patient phenotypes. As therapeutic options expand, there is an increasing demand for robust biomarkers to enhance prognostic accuracy, anticipate treatment response, track disease progression, and support personalized clinical decision-making. This narrative review critically examines the literature and discusses the role and appropriate application of key biomarkers across different age groups, ranging from presymptomatic newborns to adults with chronic disease. Genetic testing remains the diagnostic gold standard, with SMN2 copy number serving as the strongest prognostic indicator. However, substantial phenotypic variability exists among individuals with the same SMN2 copy number. Neurophysiological measures, including compound muscle action potential (CMAP) and motor unit number estimation (MUNE), accurately inform about motor neuron integrity, often anticipating clinical changes and potentially predicting treatment responsiveness. Circulating neurofilaments (NF) are increasingly recognized as sensitive biomarkers of active neurodegeneration. While NF holds promise in infants and younger children, its relevance in adolescents and adults remains limited. Conversely, quantitative muscle imaging techniques, such as MRI and ultrasound, may be valuable tools in adolescent and adult patients, capturing long-term muscle structural changes. By reviewing the current evidence across age groups, we provide an overview of biomarker application in newborns, children and adolescents/adults for diagnostic, prognostic, predictive, and monitoring purposes to help advance individualized management across all SMA stages.