We present a three-dimensional articulatory model of the vocal tract with the capability to simulate growth from infancy to adulthood. This model is intended to be applied for the articulatory synthesis of children's speech and the study of speech acquisition. To generate the vocal tract shape for a given age and sex, we resize and translate the anatomic structures of an adult reference vocal tract according to natural growth patterns. Furthermore, we discuss the transformation of the articulatory state of the reference vocal tract to the articulation for a vocal tract with a changed anatomy. The articulatory transformation was examined for the vocal tract of an 11-year-old boy by means of six vowels. To reproduce the formant frequencies measured for children of that age, it is not enough to scale the articulation (especially the tongue shape) analogous to the changes of the palatal and pharyngeal length.