For most people, it is hard enough to get good in one topic in our lifetime. Even harder to become a total expert.<p>To do this in multiple disciplines is incredible. I am fascinated by such people.
If the disciplines are related, like branches of engineering (e.g. low-level coding and rocketry in case of John Carmack), then it's not that hard. 10-15 years could be enough to get to mastery when switching branches. It's a different case when you want to switch from let's say engineering to philosophy - then almost none of the knowledge, intuitions, ways of working transfers and it could take you 20-30 years. Still, given a 75 years lifespan, should be doable.
It was a lot easier a long time ago when less was known about the various different disciplines.<p>Today it seems impossible. Though, with the right head start of a highly privileged birth, perfect timing of the right mentors, winning the genetic lottery, and other uncountable factors, you might get one or two a century. Of course, depending on the breadth of your definition.
I am somewhat of a polymath, but a narrowly focused one, in that most of my skills solely apply to computers, and very little else. But then you may ask: computers are a broad topic: how does one ever conquer the subject and attain mastery of it?<p>My answer is simple: the world of computers is an endless one, a gigantic rabbit-hole (especially when combined with The Internet). Being able to download source code for a broad number of programs and have them run and configured any way you wish is still magical, no matter how many times I've tried it.<p>Then there's the fact that you can get reliable information on an endless amount of subjects with very little friction or red tape. I am still in awe of The Internet and haven't become jaded about it yet, as many of my peers have (they literally have grown bored of the net...something I can't understand). I guess it's how you <i>apply</i> the knowledge instead of merely knowing for the sake of knowing.
The people i know with the most simultaneous breadth and depth of knowledge just inherently enjoy learning. The skills developed are more of a side effect than the goal itself. A pattern ive seen is having some sort of an anchor in different areas, like being a professional scientist but playing regularly in a band and volunteering to coach debate. Spreading areas of focus out increases likelihood of new information being interesting and relatable to existing knowledge, which makes it faster and easier to take in, especially if it intersects with multiple areas at once.
As a consultant at a small company, I have to be reasonably good at project management, sales, contracts, relationships, and my technical domain. I'm also CTO and must keep up with internal projects and client projects. Some of these require very different technical expertise<p>Maybe it's not quite what you mean, but at a small company I think you find a lot of people that while maybe not polymaths are very broad in their skillset.
If the disciplines are related, like branches of engineering (e.g. low-level coding and rocketry in case of John Carmack), then it's not that hard. 10-15 years could be enough to get to mastery when switching branches. It's a different case when you want to switch from let's say engineering to philosophy or painting - then almost none of the knowledge, intuitions, ways of working transfers and it could take you 20-30 years. Still, given a 75 years lifespan, should be doable.