As a start, I would suggest a child should get involve in some sort of musical environment at the early age like joining some playgroup. There they can sing, dance, learn the rhythm, some very basic music theory, playing with some basic instrument like small drums etc. Then as parents we may observe the child's development and whether their muscles are ready, which type of instrument the child is interested. Actually different kids are different. We may also discuss with the teachers and seek their recommendation at that time.