For most children, I recommend the beginning of first grade. By that time, a child should be able to stay by themselves for ten to fifteen minutes to practice. Of course, this is a generality. Some children who show either a strong ability to attend for periods of time or have a strong desire to play an instrument could begin in Kindergarten. The Suzuki method of string or piano instruction begins students at a much younger age but also requires a very strong commitment from the parents too.