What is a good age (or the best age) to start learning piano? This is a common question that I am asked. Well, we can accurately view music as a system of meaningful sounds that communicate thoughts and feelings. If we do this, then learning the piano is the equivalent to learning to use our vocal apparatus to also communicate thoughts and feelings (speaking). With regard to spoken language, our vocal ability is our 'instrument' used to convey such. When we choose to learn piano, we are then using the piano as our 'mouth' to communicate (thus the term "instrument").
First, before we start learning piano or how to speak, we begin learning about this type of communication in the womb since unborn children do respond to speech and music. So, the child is definitely sensing and responding to this aural stimulus and so is therefore learning about communication, language and music even thought they have no 'instrument' yet.
So, the question about when to start learning piano is more about knowing when someone (many times a child) can begin to learn speaking music using an instrument such as the piano (producing meaningful expression through the keys of the piano). And since the child began 'learning' music from the womb, obviously when they later begin learning piano, they are now becoming more of a participant in the world of musical communication that is already understood - but now in a deeper way.
To get somewhat more to the point, the initial question about what age to start piano can almost be asked in this way: What age will my child be able to learn effectively and be progressive in a formal, private one-on-one piano lesson? The answer to this question depends on each child's attention span and "maturity".
I personally do have 4 year old students that do well in a 'traditional' piano lesson. To be progressive in this context, they will need daily support at home to from their parents to learn songs and reinforce concepts.
There are piano method books that cater to the 4 and 5 year old students beginning formal piano lessons.
The above paragraphs might more address the question in this way: What's the earliest you can begin a child in formal piano lessons?
But, the real answer, I believe, to the initial question above is: Any age! To continue the analogy of likening music to a spoken language, think of it this way: If someone lacked in being fully literate with their mother language, or even of a foreign language, when should they begin to delve deeper in their understanding and ability in that language? They should begin right away no matter their age!
I personally am a native English speaking person, but I would definitely benefit and enjoy taking an English class today by deepening my understanding of grammar, syntax, classic English literature, vocabulary, poetry, etc.
In another vein, could I find enjoyment and be enriched from taking a beginning Chinese language class today? Yes! It would be more difficult since the language would be super foreign to me and I would be starting as a baby in my understanding of Chinese, but, I could find much to enjoy by taking on the endeavor.
Of course, in some senses, I think the young child beginner of piano has some advantages over the person who begins to formally study music and piano as an adult. Yet, I believe the adult beginner has some totally different advantages over the young child beginner.
So, the question of "what is a good age to start piano" is complex (dependent on some very specific factors in the case of young children) and simple (age doesn't matter). With the guidance of a good teacher, your specific situation can be assessed so that you can be confident in a good educational path.
There may be some other life situations and circumstances that could also dictate when is a good time to start. For example, if there is some particular stressful, busy project you are working on but that will end in a couple months, it could be better to wait until you have the circumstances to be committed to attending lessons and practicing. So, in this context, waiting could be smarter.
Sometimes just making the commitment to start could be good so that you force yourself to learn might be good. Again, each situation and person would need the good advice of a good teacher.
After all is said and done, I believe prioritizing music education is an endeavor that will not be regretted!