Although educational development of youths can profoundly affect their other domains of health and well-being across later life trajectories, little research has investigated the prolonged effects of family and individual contexts of youths in middle-school years, a most critical developmental and formative stage, on their educational achievement in middle-aged adulthood. The current study employed data of a nationwide representative sample of middle-school youth students in the Longitudinal Study of American Youth (LSAY) to examine how grade-7 parental support for college education, family SES, and educational expectations of youths contribute to their educational achievement in adulthood of mid-thirties through their development of grade-8 academic commitment and grade-9 educational performance in terms of English, mathematics, science, and social studies grade scores. Results based on structural equation modeling of longitudinal relationship found that grade-7 parental support for college education, family SES, and educational expectations of youths had significant and direct effects on youths’ higher educational achievement in adulthood, and youths’ grade-8 academic commitment and grade-9 educational performance significantly mediated the effects of grade-7 family SES, parental support for college education, and educational expectations of youths on their educational achievement in adulthood respectively and/or concurrently. Furthermore, interaction analysis supported the promotive but not buffering effects of grade-7 educational expectations of youths by family SES on their grade-9 educational performance and educational achievement in adulthood. Implications related to the important findings of the current study pertaining to educational development of youths are discussed.