Following the LEAP and PETIT studies demonstrating that early allergen introduction can prevent food allergies, Japan’s Ministry of Health, Labor and Welfare revised its weaning guideline in 2019. The updated guideline recommended earlier egg introduction and noted that delaying specific foods may increase allergy risk. This study examined trends in the timing of solid food introduction among Japanese infants before and after this revision. We conducted a retrospective analysis using data from the parenting application Papatto Ikuji collected between 2014 and 2023. After excluding outliers and errors, 59,433 infants were analyzed. The age at solid food introduction was examined by birth year. From 2014 to 2018, the age distribution showed two peaks at 5 and 6 months. After 2019, the 6-month peak declined, leaving a single 5-month peak by 2021. These findings suggest that national guideline revisions substantially influenced caregivers’ feeding behaviors, leading to earlier solid food introduction in Japan.