To me the closest example of an "overnight success" is where someone bursts on the scene as a celebrity because of their discovered personality. For example, people on reality shows become "niche" successful because they are discovered, rarely because they toiled away at that skill. However, their "success" still comes from years upon years of refined, unique experience to create the look+personality that created a star. If they had been discovered in the prepubescent period, it is almost a certainty nobody would have cared.

It's the same idea, just with different variables.