Let me ask this:
Take a person, start them at age 5. They practice continuously for 15 years. By the time they are 20, they should be quite proficient.
Take the same person, start them at age 20, they practice continuously for 15 years and by the time they are 35, are they as proficient as the first case?
Take the same person, start them at age 30, same thing, 15 years continuous practice. Now they are 45. Will he be even less proficient than the 35 year old?
The question then, Is it a given that if this person started at 5 that he would be more proficient than the other two examples?
What I'm getting at, Is it vital to have started very young, not only that it gives you more available years of practice, but because at this age one learns things better and faster??
No, neither from an empirical nor scientific point of view there's any reason to believe that given the same amount of time your progress better and quicker at 5
In fact, in the example above it is quite the opposite
Let me tell you way
Coordination is developed in the age range from 7 to 12, so starting at 5 would be just a disavantage in coordination problems
When you start at 5 you basically spend 1 or 2 years understanding certain musical notions that would be immediate laters because of math and movement knowledge aquired through experience and sport
When you're 5 you're likely to learn wrong movements because 90% of all teachers f the world don't know a yota about teaching a efficient movements but you're less likely to forget them because of the fast growing of your cartilage resulting in a more strong movements memory and muscle memory
If you're a late beginner you can learn bad movements, realize this, read a book about good movements, have a Taubman session and learn new correct movements
It's easier not to rely on the fallacious teaching of your teacher when you're older and you know there are alternative and you learn the new better alternative than
if you were 5 year old, were growing and had only your teacher who you think can't be wrong as your only alternative
The only advantage of being 5 or 7 is that when you're young you have not yet conformed to society fakeness and dryness, you have not yet put a mask on you and learned to repress your emotions and feelings, you have not yet conformed to the belief that like sucks and everything is hard and we're always at competition
Because of this children might learn better, because they don't believe in limitation, they never think "I'll never be able to do this" "I'm worse than him" "I sucks at piano" "I'm progressing slowly" and things like that and because of this lack of mental blocks they might learn quicker
As Picasso said "It takes a long time to become a child" or as Klee said "only children still have the power to see" so a late beginner must keep this sensation from his/her childhood, the sensation that nothing is impossible and that there are no limitation you must live and practicing in a alpha stadium like children instead of the symbolic and analitical stadium betha, living the world experiencing the sensation instead of think according to symbolism and beliefs
Think about this: there classes in conservatories where you can obtain the diploma in 5 years instead of 10
I've seen many late beginners complete the 5 years course becoming wornderful pianist but I've never seen a child getting a diploma in 5 years, in fact when they start a 5 they need more than 10 years to get a diploma usually
Daniel