Glucagon-like peptide-1 receptor agonist (GLP-1RA) has attracted attention owing to its hypoglycemic and weight-loss effects, and various dosage forms are available in the market. Recently, glucose-dependent insulin secretion-stimulating polypeptide (GIP), an incretin hormone in the same family as GLP-1, has attracted attention, and tirzepatide, a GIP/GLP-1RA, has been launched. Given the short duration of tirzepatide on the market and the fact that its therapeutic effects in patients of different ages have not been reported, we conducted this study. HbA1c improved significantly in patients aged ≥ 65 years, whereas HbA1c, weight, and LDL cholesterol also improved significantly in patients aged ≤ 64 years when compared between the beginning of use and 3 months following use. Tirzepatide has a hypoglycemic effect regardless of age; however, its weight loss effect may be more pronounced in younger age groups. Therefore, optimal diabetes treatment with tirzepatide should consider the age and weight of patients.