No, Beethoven for example continued improving his skill until near his death. Long after he'd written pieces of great genius.
Gladwell's observation was that people who achieve greatness work 10,000 hours.....which doesn't mean that you will reach greatness if you work 10,000 hours, but that if you don't, you certainly won't. More interestingly, he observes that working 10,000 hours is a greater indicator of success than IQ.
I remember reading somewhere also, that it helps to have a master teacher who knows how to guide you through those 10,000 hours. Otherwise you might be swinging with the wrong golf club for half those hours.