Background/Objectives: Serum vitamin B12 testing is frequently ordered in pediatric outpatient practice, either routinely or incidentally as part of broad laboratory panels or in response to symptoms and nutritional concerns; however, how the yield of low serum vitamin B12 results varies across age groups and testing indications remains poorly defined. The primary focus was age-related variation in low serum vitamin B12 results, with recorded testing indications examined as a secondary question and symptom-specific analyses considered exploratory. Methods: This single-center retrospective study analyzed testing episodes from children aged 0–18 years attending general pediatric outpatient clinics in 2025. Outcomes were <200 pg/mL (primary) and <300 pg/mL (secondary). Two blinded physicians classified testing indications. Cluster-robust regression evaluated age, sex, and indication; 18 symptom indicators were evaluated in age- and sex-adjusted models with Benjamini–Hochberg correction. Results: Of 34,744 episodes, 34,721 were valid; 5.8% were <200 pg/mL and 27.5% were <300 pg/mL. Low-result yield (<200 pg/mL) showed a bimodal age pattern: 7.9% at ages 0–2 years, 2.3% at 3–5 years, and 10.6% at 15–18 years. The number of tests per low result ranged from 9.4 at ages 15–18 years to 42.8 at ages 3–5 years. Routine or incidental testing comprised 63.9% of episodes; crude yield by indication ranged from 4.9% to 7.3%. Compared with 6–11 years, adjusted odds were higher at 0–2 years (aOR 2.81, 95% CI 2.44–3.24) and 15–18 years (aOR 3.92, 95% CI 3.42–4.51). No indication independently predicted the primary outcome; no symptom remained significant for that outcome after false discovery rate correction. Conclusions: The yield of low serum vitamin B12 results was strongly age-dependent and bimodal, with higher-yield periods during the first two years of life and adolescence. Broad retrospectively recorded testing indications were not independently associated with results < 200 pg/mL after adjustment for age and sex. These findings support age-informed assessment alongside clinician-directed testing.