A controlled crossover trial with 18-25 year olds found that consuming an 81% ultraprocessed food diet for just two weeks decreased orbitofrontal cortex response to milkshake rewards, while a zero-UPF diet increased this brain activity. This effect occurred specifically in adolescents aged 18-21, but not in those 22-25, suggesting a critical window of neuroplasticity vulnerability. The orbitofrontal cortex governs reward processing and impulse control, making these findings particularly concerning for long-term eating behavior patterns. The research addresses a critical gap in understanding how the modern food environment shapes developing brains during a period when many young adults gain independence over food choices. However, this preprint awaits peer review and several limitations warrant caution. The two-week intervention period may not reflect long-term dietary effects, the sample size for the age-specific analysis was likely small, and the study cannot establish whether observed changes persist or lead to lasting behavioral consequences. While the findings suggest adolescent brains may be uniquely susceptible to ultraprocessed food effects, replication in larger cohorts with longer follow-up periods will be essential before drawing definitive conclusions about optimal dietary recommendations for this age group.