Animal maturity is an important concept in the meat industry, with emphasis on producing less mature (i.e. “physiologically younger”) animals with superior carcase characteristics such as greater tenderness and lower fatness. Unfortunately the concept of ‘maturity’ remains poorly defined. Where specified, ‘maturity’ usually refers to some proportion or index expressed relative to the mature state, in which the animal is in anatomical and/or compositional equilibrium. However indices of maturity referenced to body weight or composition (e.g. muscle:bone ratio) are problematic for assessing genotypic effects in modern prime lamb production, where terminal sires may be selected for specific compositional traits such as rapid muscle growth or low body fat. In such cases it may be preferable to define other indices such as skeletal maturity, by staging development in relation to longitudinal bone growth and mineral maturation and thus lead to a better understanding of musculoskeletal development in sheep.