This review critically examines training and testing strategies in youth athletes, focusing on growth, maturation, and specialization. Early specialization, occurring in athletes younger than 12 years, is associated with training volumes above 15 h/week and with dropout rates of nearly 30–35% before age 15. The aim was to evaluate age- and maturity-specific protocols, identify limitations of adult-derived models, and provide evidence-informed recommendations. A structured search in PubMed, Scopus, and Web of Science between 2000 and 2025 identified 325 articles; after screening, 87 were included, with 19 addressing maturation, 16 testing, 18 training load, 15 motor skill development, and 19 specialization. Results show that growth variability, particularly during peak height velocity (10–12 y girls; 12–14 y boys), increases injury risk two- to threefold. Standardized adult testing (e.g., VO₂max treadmill) is often unreliable in adolescents, while field-based protocols such as the Yo-Yo IR1, CMJ, and FMS provide safer and more valid alternatives. Flexible periodization with deload cycles of 1–2 weeks and monitoring through RPE, GPS, and HR improves both safety and adaptation. Diversified sport participation further reduces burnout and supports long-term outcomes. Integrating maturity status into training and testing optimizes development, minimizes injury, and sustains lifelong athlete engagement.