The difference in play?

Why do people say you have to start really young to be an expert pianist?
They don't sound too different to your regular piano player to me.

eg. What's the difference in play between Elton John and a so called expert pianist.

Regular?

They say it because you'll have more time to develop your skills if you start when you're young. Also, there's something about being young that makes it easier to learn things.

The piano is an instrument that is easy to learn but nearly impossible to master. Starting anything earlier will give you an advantage at learning it faster than people who start it later in life, but just because a child starts learning piano early doesn't mean they will become excellent at it, and just because someone starts it later in life doesn't mean they will be bad at it. A lot of multi-instrumentalists in the music industry actually pick up piano as a second instrument, even though it is regarded as a "base" instrument for learning others.

How many experts pianists have you heard and who were they ?

You don't HAVE to start really young to be an expert, and you don't necessarily do well once you start early. Everyone has differing degrees of ability. But it's easier to learn it well when you're younger. That's why many so-called 'expert' pianists started when they were kids. 'Expert' pianists definitely sound quite different to 'regular' piano players, and a musically-trained ear will be able to tell the difference between really great pianists and more amateur ones. It's not about the accuracy of notes, anybody can learn the notes and play it accurately, but it's about the musicianship, sensitivity and maturity in their playing. And by maturity, I don't just mean in terms of age, but also in terms of how they interpret a piece, how they convey the music across to the audience, etc. When you start young, it's easier to master the technical side of piano-playing because children's minds absorb new things much faster than adults - they don't know what's hard and what's not, they just do what they're told. This is why you see 'child prodigies' who are more comfortable on the piano than some people twice or thrice their age. Plus, you've more time to master the more difficult techniques and develop musical maturity before you go past the 'golden age of technical training' (usually around ages 13-18). It's just something queer that our brains do to make those our most absorbent years, in addition to ages 3-7 which are the most absorbent years in children.

"eg. What's the difference in play between Elton John and a so called expert pianist."
Expert in what kind of music? Classical? It's a huge difference between Elton John and... let's say... Dinu Lipatti (he's from my country and I'm proud with him ;) ). Elton John plays a music that is build on a few melodic lines, with a pretty simple harmonic background. A "so called expert pianist" plays a music in which every motif has it's own development, a "so called expert pianist" must know much more stylistic facts about many pieces, while Elton John plays his own music. A "so called expert pianist" must have a much higher and comprehensive level of music than Elton John.
But why am I trying to do? There's just one single fact that approach Elton John by a "so called expert pianist": the fact that both of them are using the piano to express their fillings.