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The 305-d yield records of 1,048,942 Italian Holstein-Friesian heifers were used to determine the effect of age at first calving on milk yield and fat and protein concentration. Research showed that there is a positive effect of age at first calving on milk yield and fat percentage and a negative effect on protein percentage. In six scenarios of milk price and rearing costs (2 × 3, respectively), the effect of age at first calving on the difference between milk yield returns and rearing costs was calculated. We concluded that the most positive difference was achieved with age at first calving between 23 and 24 mo.