Given that he started publishing games before he was 20 and had produced Commander Keen, Wolfenstein, and a couple of Dooms before he was 25, I'd guess he was well on his way in his teens.

Not to say that success correlates with skills, I suppose, but he's fairly well-regarded as an innovator from an early age. Odd choice of example from the article's author.

He started releasing games at 19. So probably pretty good.

I mean, hell, I'm 19 now and a professional web developer. I'm not exactly a "teenage coding god" but I'm definitely good at programming.

The trope exists for a reason. It's not beyond imagination (although it is unrealistic, just like any other trope).