While it's true that people start their careers early in life, this may also be an indication that they simply have a strong proclivity towards that field. There have been longitudinal studies, for example, that demonstrate that when kids do play-based learning in kindergarten vs hard core academics, they do as well, if not better at academics later on. There have even been studies that show it's better to delay kindergarten for some kids. https://pz.harvard.edu/projects/pedagogy-of-play

The other danger is that forcing a kid to learn a skill they're not interested in early can turn them off to it all together.

I haven't done a ton of research on this, but I do know that often when kids start early, they can also burn out (look at child actors as an example). The only area I've seen research on a real difference is children who learn languages before age 5 (and how they process the languages in their brain)

There are some amazing developers who learned to code as adults.

While it could be true, I'm pretty skeptical about age five versus age six being the difference your friend didn't become a world class pianist (sounds maybe like an excuse for not having the talent, discipline or simply the luck, which does play a huge factor). Starting early may help their technique and memorization to some extent, but afterwards judging a musician is highly subjective, and even someone who plays well at one moment in history, may not at another.