Between the ages of 18 and 21, the voice stabilizes because the vocal cords and larynx have reached their full growth. While there may be some changes by age 30, most people end up with physical changes due to hormones by age 21 or something like that. This process can take up to a year. Usually, by age 17, the voice completely stabilizes.
If a teenager's voice has not changed at that time and other secondary sexual characteristics have not been developed, there may be hormonal problems. A lot depends on your gender. Male voices generally complete most of the development in their early twenties, female voices in their late twenties or older. It also depends on the size of your voice.
As a rule, larger voices mature more slowly than younger voices. Everyone's time is different, so some children's voices may start to change sooner and others may start a little later. Usually, a child's voice begins to change between the ages of 11 and 14 and a half, usually right after the big growth spurt. Some children's voices may change gradually, while those of others may change rapidly.
In the early 1920s, a healthy voice like the rest of the body normally shows an exciting combination of strength and flexibility. Unfortunately, this peak in range and agility typically decreases slightly in the late 20s and early 30s, when one considers that the voice is fully biologically mature. Keep in mind that this is almost the same time that the frontal lobe of the brain completes its development, offering you a more reliable and thoughtful good judgment than your adolescent brain allowed. Early adulthood is a milestone for the singing voice because the voice box develops fully and the voice that sings can reach its full potential.
If you want singing to be your career for life, the idea of losing your singing voice when you grow up can be stressful. Growing old can take your singing voice by leaps and bounds because familiarizing yourself with your vocal ability and working on your technique can help you sing better.