I started playing the keyboards at the age of six and a half. I only could practice on a small, 61-key Kawai board since digital pianos didn't exist yet and my family could not afford an upright. I received the training of a concert pianist, but fell in love with "outside" styles like jazz, funk and blues, so I pursued my classical studies alongside those as well. This is not something that sits well with many devoted classical artists, since it can be seen as a distraction from the music that "matters". To them I simply point to Keith Jarret, a renown pianist of both classical and jazz styles. Anyway, let's get back to the point.
Starting an instrument young is a rewarding experience, but it comes with great sacrifice. Many children of exceptional talent I have read about or seen on YouTube all have the same biography: eight hours of practice a day, six days a week and lessons with a very expensive instructor at the nearest conservatory, even if it requires a long commute. These kids have few, if any, friends, have no social life and are actually thinking like adults. Their childhood has ended before it even began. There is nothing wrong with studying music young, but when does the line get crossed? Is it not cruel to force a child to become a socially awkward, borderline hermit just for the sake of art? Even if they are enjoying it, is it healthy? For many of these kids music is more of a drug than a hobby or career path, and a day without practice causes deep remorse and depression, like the mental equivalent of the dope-sickness that comes with heroin withdrawal. That is in no way healthy for any child.
One of the biggest reasons that so many instructors urge that children start playing the piano as soon as they can reach the keys comes from the idea that their muscles will be trained better and their hands more dexterous. However, what these instructors neglect (they are music teachers, not physiologists after all) is that muscle development must be done slowly and over time. You can't just assign a youngster four major pieces and tell them to have them ready for next week. Muscles grow through slower exercises. When using weights, bodybuilders will use lighter dumbbells and more reps at a faster pace for toning muscle. To build strength, they use heavier weights, less reps and a slower pace, gradually adding weight over time. Children cannot handle that much exercise because it can damage muscles and joints, this includes piano or any instrument exercises.
I have found that in recent years, many people who started studying at an older age, between twelve and sixteen, not only have more dexterity at their instrument, but far more technicality and artistic integrity than their peers who started much younger. They are also ready for higher level pieces sooner than the ones who started earlier. Why?
One of my theories is that kids today are learning to type earlier than ever. I didn't learn how to type at a computer until the 7th grade when computer classes started. Today, there are kids in kindergarten that can out-type my generation by dozens of words per minute. The technique for using a computer keyboard has similarities to the piano and other keyboard instruments, so the proper hands and dexterity are there long before an interest in music may fully develop.
With many of my older students who started in their teens, they have another similarity; video games. These students have very high level technique and can tackle level 8, 9, and in some cases, 10 pieces despite only having played three years. The one thing they all have in common is that they played or play video games. The act of working a controller, and the hand-eye coordination it developed, has greatly improved their technique despite not having rigorous training right out of the crib. Granted, this is no excuse for actual practice, but it really defeats the idea that only those who start in diapers will have the talent needed to do anything with their instrument.
The greatest reason for this thinking today is the myth of the child prodigy. No child is born with the ability to play an instrument (or produce great work of any nature on their own, keep those last three words in mind). Those we see as prodigies are more often than not heavily pushed to practice to the point of obsession. Mozart was pushed by his own father until he was molded into the legend he is today. His father never gave him a chance, he was forced into music and was forced to keep it up once it started. When we see kids up on stage in classical piano competitions, they are there only because they have been pushed to become what they are, whether of their own will or not. Many who claim this is what they want probably only believe that because they have never had the chance to try anything else in life, or they unwittingly want their parents' or teacher's goals realized through them. You go out there and find me one kid who doesn't want friends, or to go to parties, or to relax and enjoy their childhood of their own free will. They don't exist.
These children may oftentimes have the technicality of the works played, but are all too often lacking in the emotional responses to them. It's no doubt that the reason for this is that they haven't lived long enough, or have had life outside of practice to experience things that can be reflected in the music. This is why so many musicians continue their training after leaving the conservatory and study with the great masters; their playing lacks a story, expression and soul. They can play the notes, but they can't feel them! This is something that cannot be taught either, it's something that only comes from living a life full of experiences, and what life experiences can someone who's been trapped in their practice room for twenty years have?
Music studies must be done alongside other activities, even if your goal is to become a concert pianist or orchestra member. No matter how much you practice, you need to be allowed to live in an equal amount of time, no matter the age! Even if your child starts lessons at three or four, give them a chance away from the instrument. As a professional, I practice piano and bass in equal proportion and max out at around four hours a day (I alternate each day so I do piano one day and bass the next so give me equal time). Five hours is the maximum I'd recommend anyone to practice, especially in children. After that, the muscles and joints become overworked and deprived of fuel, leading to damage and even arthritis. The fact remains that practice makes perfect, as cliche as that is, but too much practice makes problems. If you want your music to be anything that expresses the deepest part of your soul, you need to be able to find that soul and how to speak through it with experience.
There's a big world beyond the practice room, and the music that happens in there can only grow through what's out there.