Below is an excellent article from the Music Together® "Mothership."
My shorter answer is that the work we do (modeling, engaging, supporting exploration and review) is the most developmentally appropriate method for helping children attain a Basic Music Competence ("BMC"--marked by accuracy of tonal and rhythmic expression) before the age of 6. Before we ask a child to channel musicality through an instrument and learn theory which is putting labels on intuitive concepts, we need the children to have developed that musicality and intuitive understanding of musical concepts. An instrument is a tool for expressing musicality. You can't use the tool if the musicality is underdeveloped. The labels can't hang on ideas that are not yet grasped. The foundation has to be there before you can build on it. So ask yourself, has my child attained BMC?
If the answer is "yes", if your child is singing in tune already and moving pretty accurately to the beat, then you need to assess social readiness. More on that below.
Most important, if you are starting a child on formal music lessons before the age of 8, assess your teacher's understanding of your child's age and stage of development and make sure she knows what is developmentally appropriate for your child. Don't ask a 3 or 4 year old to sit still like an 8 year old for 45 minutes (unless your child has a very unusual temperament ;) )
Formal music study is one of the best things you can do for your child! Making sure your child is ready sets the stage for happy - so worthwhile! - life-long learning.
From the Center for Music and Young Children:
Today's cultural norm seems to be a desire to produce children who are smarter and more accomplished at ever earlier ages. Starting with music-in-the-womb programs for the prenatal set, parents are bombarded with enrichment options for the very young, and it's hard to judge what is actually developmentally appropriate for your growing child. Is earlier really better when it comes to traditional music lessons? Will your child somehow be "behind" if she waits until age six to begin studying piano? In point of fact, music lessons can all too often be a frustrating and even painful experience for a child who is not developmentally or musically ready for them; and most children—even the obviously musically inclined—can benefit from waiting a little longer to start traditional lessons than the anxious parent might be given to believe.
To have a good chance of enjoying and embracing formal music study, a child must have a readiness that goes beyond the traditional prescription of being able to sit still for fifteen minutes, count from 1 to 5, and know the letters A to G. It's important to consider your child's temperament, physical development, and level of tonal and rhythmic competence before signing him up for lessons, and to ask yourself what you as a parent want for your child.
It helps to be clear-eyed about the nature of traditional instrument lessons. They involve learning to read music, which is a complex cognitive process, and they require a high degree of hand-eye coordination. They are inevitably product-oriented, focused on semi-regular recitals at which the student's learning will be displayed and judged. Reflect on whether your young child is developmentally ready for the pressure of performing a piece in public. Practicing a piece to become performance-ready requires persistence, patience, and commitment, and music study will not magically produce these qualities in a child who is not already showing some sign of them. If your child is easily frustrated and has little patience for repetitious tasks, it's probably better to continue for a while longer with non-formal music and movement experiences like Music Together classes.