Like learning a language, a person can begin to study music at any age. But the results will differ. A child who picks up a language at 4 or 5 will eventually sound like a native speaker. An adult may be able to learn several words right away and communicate much faster than the child, but will rarely sound convincingly native. It is the same with music.
A 4 year old may take months to learn what an adult student can learn in a few lessons. Because of this, parents sometimes say that it is better to save money and wait until the child old enough to learn more quickly. But the 12 year old is less likely to know and understand inside and out what she’s learning. She will have a harder time “sounding native”. There’s a reason most of the world’s greatest pianists and musicians started young.
According to the Suzuki philosophy, a child should begin at age 0. How? Merely by surrounding the child with music from the day of his birth. Just as a child doesn’t begin to speak until he has heard his mother tongue for a few years, then he should be frequently exposed to music for his first years before formal lessons begin. But most parents aren’t this engrossed unless musicians themselves.
So formal lessons should be started as soon as the child is capable. A twenty minute lesson is great for a 4 year old. By the age of 6, the child should probably be having 30 minute lessons. The student doesn’t need to be able to focus for 20 minutes to begin lessons. Piano teachers can use frequent activity changes and other tricks to keep the student focused. But every child is ready at a different age. If the child is able to tell left and right apart, count to 10, and focus for 5 minutes in a row, she is perfectly able to begin lessons.