Methotrexate is a medication that is used to treat a variety of medical conditions, including cancer and autoimmune diseases such as rheumatoid arthritis. The dosage recommendations for methotrexate in younger patients depend on the condition being treated and the age and weight of the patient.

According to Drug Patent Watch [1], the recommended dosage of methotrexate for pediatric patients with acute lymphoblastic leukemia (ALL) is 3.3 mg/m2/day for maintenance therapy. For patients with osteosarcoma, the recommended dosage is 12 grams/m2 over 4 days, followed by leucovorin rescue.

A study published in the Journal of Clinical Oncology [2] found that the recommended dosage of methotrexate for children with osteosarcoma is based on body surface area and ranges from 8 to 12 grams/m2. The study also noted that the dosage should be adjusted based on renal function and the presence of other medical conditions.

A review article published in the journal Pediatric Rheumatology [3] noted that the recommended dosage of methotrexate for pediatric patients with rheumatoid arthritis is typically between 10 and 20 mg/m2/week, with a maximum dosage of 25 mg/m2/week. The article also noted that the dosage should be adjusted based on the patient's weight and renal function.

In summary, the recommended dosage of methotrexate for younger patients depends on the condition being treated, the age and weight of the patient, and other medical factors such as renal function. Patients should always consult with their healthcare provider to determine the appropriate dosage for their specific situation.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/METHOTREXATE
[2] https://ascopubs.org/doi/10.1200/jco.2015.62.4544
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8349527/