A cross-sectional twin design was used to study the developmental nature of genetic and environmental influences on height, weight, and body mass index. The sample of same-sex adult male and female twins consisted of 586 monozygotic and 447 like-sex dizygotic twin pairs aged 18 to 81 years. Means and variances suggested normative age differences for all three physical variables. Biometrical model-fitting with maximum likelihood methods of parameter estimation indicated that the general best-fitting model across the age groups for height, weight, and body mass index was one in which the genetic effects were additive and the environmental effects were from nonshared, idiosyncratic experiences. The best-fitting cross-sectional biometrical model for height, weight, and body mass index indicated that additive genetic variance remained stable while nonshared environmental variance increased with age. This increase in environmental variance but stable genetic variance resulted in decreasing heritability with age for height (heritability ranging from 0.89 in the youngest group to 0.87 in the oldest), weight (heritability ranging from 0.86 in the youngest group to 0.70 in the oldest), and body mass index (heritability ranging from 0.82 in the youngest group to 0.63 in the oldest).