Subcutaneous fat (SF) and Longissimus thoracis et lumborum muscle (LM) depths of growing lambs from six breeds were monitored. Ram and ewe lambs from Dohne Merino, Dormer, Dorper, Meatmaster, South African Mutton Merino and White Dorper breeds were reared together under optimal feedlot management conditions and were subjected to ultrasound scans every fortnight from the point when the lambs attained 20 kg body weight up until 300 days of age. Exponential and linear functions were fitted to describe the change in SF depth, using body weight and age respectively, while the power curve was fitted to describe the change in LM depth. Moderate to good fits were achieved for the models describing the increase in SF depth of the various production groups with weight (R2 = 0.48–0.72) or age (R2 = 0.53–0.78) as predictor. These curves showed that hair sheep breeds are early maturing relative to wool sheep breeds, and should thus be slaughtered at lower live weights to achieve uniform levels of carcass fatness.