This question creates a lot of discussion among piano teachers. There are as many opinions as there are teachers. There are many different methods aimed at different ages and different learners, regardless of age.
Some things to consider in answering this. If the student is really eager (no matter what age), lots of musical learning can happen when music is listened to, moved with, sung along to, clapped and tapped along with. From when a parent plays or sings a lullaby a child takes it all in.
At preschool age, around three or four, many kids love the group musical experience like Mini Maestros, Kindermusik and the like. These are great starters for learning a love and appreciation of music. A lot of these general musical activities will be used in lessons and can be used at home.
Once a child reaches school age (six years and older) has worked well for many a student to start more traditional piano lesson format as a lot of the basic concepts of listening and sitting concentrating, the ability to read numbers and letters and understand another adult's descriptions are beginning. These skills are very helpful in learning a musical instrument of any kind - but especially piano.
A child can start piano very young, but progress is slower in actual reading of the music and playing as there is a lot of musical language involved in the playing which needs to be gradually acquired. Lots of musical activities on and off a piano bench will be engaged in and less sitting still listening and learning. Some argue that training very young ears is best, other teachers will not happily start on piano lessons until at least seven.
When a child starts around seven or eight, progress is much more steady on the traditional path of learning to read music and playing what is written on a score. And every year over this, seems to result in a more rapid uptake of the basic ideas and concepts presented.
In many instances a student starting at five and a student starting at eight will arrive at the same piano playing ability level by the time they are around ten, depending on the teacher and the student.
