This chapter provides a comprehensive overview of cognitive development, cognitive decline, and evidence-based interventions across the lifespan in individuals with Autism Spectrum Disorders (ASDs). It begins by examining the developmental heterogeneity characteristic of ASD, highlighting diverse trajectories in cognitive, language, social, and motor domains, and emphasizing the importance of early cognitive developmental assessment using tools such as the Bayley-III, Wechsler Intelligence Scales, Vineland-II, and KABC-II. These measures support individualized educational and therapeutic planning by identifying specific strengths and challenges. This chapter then reviews key intervention models, including Early Intensive Behavioral Intervention (EIBI), play-based and parent-mediated interventions (SCERTS, DIR/Floortime, PLAY Project, and Hanen’s More Than Words), as well as behavioral, speech-language, occupational, and social skills approaches, summarizing their effects on cognitive, adaptive, and social-emotional outcomes. A detailed discussion of sensory processing, motor learning, executive functioning, social cognition, and theory of mind further illustrates the multifaceted nature of cognitive development in ASD. Finally, this chapter explores theoretical models of cognitive aging in autistic adults, including the safeguard hypothesis, accelerated aging model, and parallel development theory, integrating emerging findings on neurobiological correlates and age-related cognitive changes. Collectively, these sections underscore the need for developmentally informed, individualized, and lifespan-oriented assessment and intervention strategies to support optimal cognitive and adaptive functioning in individuals with ASD.