This long-term longitudinal study addressed the theoretical underpinning of academic intrinsic motivation (AIM) from childhood through adolescence, to need for cognition (NFC) and educational attainment (EA) during adulthood. AIM was measured from 9 to 17 years old, NFC and EA at 29 years old, and IQ at 8 years old. Latent change and growth mixture modeling were utilized. These models complemented each other, revealing that initial motivational status significantly related to both outcomes. Growth mixture modeling elaborated the findings by identifying distinctive subgroups in initial status and developmental change. In contrast to children with initially higher AIM, those starting lower declined resulting in lower NFC and EA. IQ was controlled in these analyses. Findings enhance understanding of trajectories across two decades of development, indicating that students' early motivation relates to adulthood NFC and EA. Implications for educational practices are discussed.