Time is an equally important consideration in developing, re-coordinating, and increasing the tonicity of vocal muscles. Tradition has it that a period of seven to nine years is required (even for the acknowledged master teachers of the Bel Canto era) to bring a voice to its fullest state of development. Furthermore, full vocal maturation is not reached until about the age of forty. Provided that one sings correctly, has not abused the mechanism and maintains good health, however, the singer’s vocal prime should last at least twenty years – from age forty to sixty.
Reid, Cornelius L. A dictionary of vocal terminology: an analysis. Joseph Patelson Music House, 1983.