You can, however, be too young for vocal development. Pushing to develop immature vocal folds can cause them permanent damage. It can be very hard, if not impossible, to learn to compensate for that damage later on.
That’s right; compensate for, not recover from. Damage to young developing vocal folds can leave scar tissue that will cause a permanent dead-zone in the singer’s tonal range. Some cases may be helped somewhat by surgery, but most will endure.
The best age to begin developing the voice is the mid to late teens. By that age, the vocal folds will be nearly matured and boys voices should have changed and become more stable. That having been said, careful and conservative vocal training could be very good for younger children.
By “conservative”, I mean that there should be no effort to develop the vocal range, to shape, or to build the voice of a child. There should be no effort to make a child’s voice go beyond its natural, comfortable range. A child should also not sing particularly loudly.
Vocal training for a young person should consist of singing in their natural, comfortable range, while being coached in maintaining good posture, breathing techniques, and good general and vocal health. They can practice matching and maintaining pitch, harmonizing, reading music, playing an instrument, and a number of other things that play a part in being a good vocalist. They should, however, not try to develop their voice.
Singing, for a child, should be fun, natural, and relaxed. It should not be work. The child should sing age-appropriate songs and should sound like a child singing. They should not try to sound like an older vocalist.
Children, who enjoy singing, should be encouraged to join their school’s Glee Club, their church or synagogue children’s choir, or other community children’s choral group. Be careful though, that the director is not trying to push the children into other than their natural and comfortable voices. A choral director will want to fill out the vocal parts of his choir and natural Basses and soprano’s can be rare in children.
So, will I automatically refuse to advise anyone under fourteen? No, but I may be hesitant, and I will expect a great deal of parental involvement and monitoring. I will be very unlikely to agree to advise anyone who is less than ten.