My guess is it's about how curious about new things you were when computers got popular. Computers are, compared to a lot of real world devices, a lot more complicated than pretty much any consumer tool that predates them, but also more forgiving. Most consumer appliances before computers had at most a couple of settings, all of which were clearly defined and handed to you. Anything beyond that involved aggressive tinkering that may or may not go well.
Computers, on the other hand, have interfaces that are vastly larger than any human brain could hope to contain. So they require exploration. This comes naturally to children, less so to fully matured adults, and is extremely rare in people middle-aged or beyond. So you get a line based on how old they were when they first got their hands on a computer.