There’s a lot of good answers here, but, since I personally qualify as a former IMO Olympian who didn’t become a mathematician, I’ll add my personal experience.

I was a bronze medalist in ’93 and a silver medalist in ‘94, which naturally led me to start my Masters right after the IMO. I had the priviledge to study at IMPA, under the great Jacob Palis, and alongside fellow Olympians such as “Gugu” Moreira and Fields-medal winning Artur Avila.

Since, to get the Masters degree, I was required to have a college degree, and realizing that doing undergrad courses in Math would’ve been a waste of time, I’ve decided to get a degree in Computer Engineering in parallel with my Masters in Math (Dynamical Systems). In ’96, at the young age of 19, I had finished my Masters and started to work on my PhD at IMPA, at the same time as I continued with my CE course, from which I graduated in ‘98.

For those of you who aren’t old enough to remember, the late nineties were known as “the dot-com bubble”, so, right after graduating, some colleagues and I immediately founded an IT consulting startup. For the first couple of years, I could manage to work on both my company and my PhD, but it was becoming increasingly clear that one of them would have to give. So, the reality is that, as others have said, I haven’t really “failed” at becoming a mathematician, but rather succeeded at having other carreer options that I found more fulfilling.

Although I deeply enjoyed every minute I spent at IMPA, I don’t regret at all the decision to have chosen the entrepreneur/IT executive life over being an academic. Now that I work with BI & Analytics, I can still be somewhat close to some cool (applied) Math, but much closer to the actual effects on business and people’s lives (and, of course, a substantially bigger pay check), which is for me a much better package than I’d have as a pure researcher.

So, TL; DR; being an Olympian usually means having several career options available, and the tools to succeed in them; being an academic mathematician is just one of them. In my own experience, most of my fellow Olympians that decided to go to the academic path became very successful (some all the way up to winning the Fields medal), and most of the ones who decided otherwise were also mostly successful in whatever career path they’ve chosen.