There may be defined age for growing sheep? Is there any difference in crossbreds or geographical variation? In fact people are using adult sheep and calculating average daily weight gain in them, is that wise?
As in any quantitave trait of animal growth is affected by the genetic makeup and the envoronment and growth is started from conception till it reach the maturity. After birth the relative growth rate is high and then decline toward maturity mainly because of deposition of fat and the growth will depend on the breed and mainly the feeding offered to the lambs