Which is almost 20 years after he began writing (according to your link). The point is, he didn't start writing at 45. Not that you're implying that, but I see plenty of pieces about people making it big when they're older, but those same pieces fail to shine as bright a light on the years and years of toiling and craft-honing that led the person to that moment.
https://en.wikipedia.org/wiki/Roald_Dahl_bibliography