K.H.
Hi! I am so pleased to hear your child is musically inclined. I am a music teacher and have started children from all different ages. It all varies with children development. There is a program called Musikgarten which helps pave the way for little ones to gain interest and training in basic music while moving up to instruments at the proper time. In the past, I required this class (some rhythm instruments and singing, but much more) to see when the window opened for more formal training. Once the attention span and certain motor skills have developed, the Suzuki Method is recommended. It starts children as early as 2 or 3yr. Again depending on each child....some wait until they are 4 or 5. This method includes the parent in the lessons to help at home with practice and proves to be very fun. Several of my students started piano or violin at 3 or 4yr and are great musicians today enjoying what they do!
Hope this helps and feel free to let me know if you need more details.