Many age estimation methods are based on growth of the dentition and skeleton. Growth follows a consistent sequence, broadly comparable between different individuals, and clear-cut changes occur over a short period so, even taking into account variation within species, age can be estimated with reasonable precision from the state of development. Growth unfolds in different ways for different parts of the body. General or somatic growth is rapid in foetal and early postnatal life, but then slows until the ‘growth spurt’ of puberty, during which any marker of overall body size increases rapidly. On the attainment of sexual maturity, growth rate falls back as the individual reaches adult size. Genital organs and most secondary sexual characters grow little before puberty, but very rapidly during the somatic growth spurt. Neural structures, by contrast, grow rapidly before birth and during infancy, after which they grow more slowly. Teeth and jaws have their own pattern related to the two dentitions. The deciduous dentition is associated with the shorter face or snout of young animals. Establishment of the permanent dentition is accompanied by elongation of the snout and the eruption of the last teeth marks attainment of adult face proportions.
Skeletal structures obey one or a mixture of growth patterns. Long bone length follows the somatic pattern, whereas the skull is a complex mixture of growth curves (Moore, 1974; Osborn, 1981; Goose & Appleton, 1982; Enlow & Hansen, 1996; Meikle, 2002).