The author does imply that great teachers have a lot to do with it. At a later age, when one is out of school or almost out of school, one obviously needs to show more personal initiative. And a lot of people here and elsewhere have asked or written about how to best self-study mathematics. However, the importance of having a teacher is often not mentioned. Even if you are very self-motivated, a teacher can make an enormous difference by showing you the cool stuff that you might miss on your own. One can be very motivated about building a house by themselves, but might have only ever seen mud bricks, and hence will try to diligently build a house out of mud bricks. A proper teacher can illuminate and guide a student through the vast world of other things out there, just like someone trained in construction can show the self-motivated builder how to build a proper house.
Now, how does one find a teacher, you ask (especially when one has left school)? A few options I can think of:
* Go back to school.
* Participate in an appropriate online community.
* Reach out to professors or even students in nearby schools.