I recently attended a Joey Alexander concert at SFJazz and was amazed how mature his playing is at his current age, given my standard of 'good musician' is rather high as a professionally trained jazz musician myself. He started playing jazz at age of 6 and started performing professionally when he was 9.

I agree that people need SOME time to learn the ropes but with proper training, 2 years is enough to be good. Note that proper training doesn't mean practicing certain technique, scale or etude hundreds of times. Nowadays there are systems designed over the years by schools such as Berklee effective enough to make someone a good jazz musician within 2 years, similar to many athlete training systems. No system can make legends, but decent musicians for sure.