There is a difference between formal vocal lessons and singing for fun. Singing in childhood helps to develop aural and rhythm skills. I started singing to my children invitro and never stopped. They could sing in harmony by age three. By five my youngest had developed an amazing sense of pitch and could play an instrument by just picking it up and blowing the correct notes. Singing helped them develop a great sense of pitch and rhythm. (Steady beat is one of the most important skills children should develop.)* I also believe it also helped with creativity.
For formal lessons on average, (for most children,) age 11 is a great time to develop proper vocal skills. The voice cannot be pushed though as it does not fully develop until late teens or early twenties.
Having said this, proper singing can be developed at any age. I have students that did not start lessons until mid to late adulthood and have developed to a high level. So age 11, in my opinion, would be the earliest to begin formal training.
Performing in musicals, singing in school choirs, taking lessons and possibly working toward competitions are all important.
Several of my voice/choir/theatre students have gone on to perform on Broadway and one has a Tony. One of my students is a prolific folk singer and has many awards and albums to her credit. I have had students go on into opera and perform at La Scala or the Met. Several are musical theatre/pop/jazz/blues singers and instrumentalists and make a great living doing what they love.
Music reading is so important if a student is serious about being a singer. Developing a sense of pitch, intonation, breath support, tessitura, and rhythm are all important. However, being able to read those rhythms, pitches, dynamics, languages and other symbols are key to being a good musician. I highly recommend serious students study voice and another instrument like piano, guitar or both in order to learn to read music, understand music theory and truly become a fine musician.
Laurie Beth Frick
*Steady beat – Researchers link ability to keep a beat to reading and language skills
https://news.northwestern.edu/stories/2013/09/the-importance-of-keeping-the-beat/