Shinichi Suzuki did not start out as a violin teacher. His father was a violin maker but he did not actually start learning how to play until the age of twenty.
To me, this is hugely significant when trying to understand a method of teaching that prides itself on being able to work with very young children. The founder of this approach to teaching was not some child prodigy, he was an adult beginner.
I think this speaks volumes for the power of time and consistency. I've lost count of how many times an adult has told me it's too late for them, they're too old to start. Yes, there are advantages to starting a child young. Areas of the brain are activated that would otherwise be sealed off. But this doesn't mean an adult can't learn.
If a child starts music at the age of four and sticks with it, there is no question that they will be a decent musician by the age of twenty-four. But let's be honest here: anyone who devotes twenty years of serious study to anything will certainly become proficient in the task.
The problem most adults have is an unwillingness to be a beginner. A child is a beginner at just about everything so it's only natural that they should be allowed time to learn such a complex task. As we age, we naturally lose that openness to learning. By the time we are forty or fifty, society demands that we need to be an expert in something if we want to have a career. This means we need to forsake trying to learn everything for the sake of learning one thing well.
This doesn't mean that we lost our ability to learn. It means we lost our willingness to learn. That's a big difference. I think Suzuki is proof of the fact that anyone can learn how to play. Learning must actually take place, however. It is the learning that leads to knowing.