Goats should have specific weights at a certain age. Age and size proportions are important when selecting your breeding stock. Goats usually reach their full mature size at three years of age. If the animal is of a specific breed, you must be familiar with the breed standards to assess for size and age.

Source: E. A. B. Oltenacu, http://www.ansci.cornell.edu/4H/meatgoats/meatgoatfs11.htm