Articulatory models reported in the literature are generally meant for a prototype male speaker. One such popular model is that proposed by Mermelstein [J. Acoust. Soc. Am. 53, 1070--1083 (1973)]. In this paper the scope of Mermelstein's model is extended to adopt it for any male speaker as well as for female and child speakers. The default vocal tract shape of the model for a given vowel is determined. The axis of the vocal tract is divided into a number of sections. Vocal tract length as well as section lengths are determined. To adopt the model for a male speaker with a vocal tract length different from that of the model, the section lengths are uniformly scaled. To adopt the model for a female or child speaker, the section lengths are scaled disproportionately over the pharyngeal and oral cavity to attain the speaker's vocal tract length. The disproportionate scale factors are based on the anatomical data. Accuracy and application of the extended model are discussed.