Wow. I spy a future life of them writing high-karma answers in vim.stackexchange.com. Much like playing a musical instrument, I feel that the only way to really master that tool is to starting using it insanely young. It might cause a hell of a shock when they go to school and get indoctrinated^H taught painfully and slowly how to edit "a document" using Microsoft Word, however...
Side story: my dad was a professional pilot, and if you replace "vim" with "IFR flight and approach planning", you get something of a similar ancedote with my life. He had an early flight simulation tool or two on a Macintosh LC, with a throttle quadrant, rudder, and yoke he built himself. I have many happy memories of trying to practice, e.g. carrier landings in an F/16 simulator, as a really quite small child. For my sixth birthday he took me up in a Piper Cherokee with a friend who was also a flight instructor, and I held straight and level flight and the long final. The colleague of his, sitting next to me, couldn't believe what the hell he was looking at. A lot of these things are just learnt fine motor skills and if you have a compliant and, above all, interested child, they pick them up very quickly. (My classmates didn't believe me – I had to wait until the photos he took on a film SLR were developed!)
> I held straight and level flight
I've decided on this: The computer is a tool. As soon as they could use a hammer, or a computer, or a match safely - and demonstrate that they know - then they could use them. My two daughters never got the hammer, my son got it at 5. And he knows why we call it the "thumb seeker".
The "computer" tool I allow for _creating_ content at about 3 to 5, I don't remember exactly. They were all able to create content (or write in VIM, yes, VIM) by 5 I think. And they were able to stop when I tell them to... So I generally didn't have to tell them to.
The "computer" tool I allow for _consuming_ content started at about 8 or 9 I think for the girls, when they demonstrated - and continue to demonstrate - that they can occupy themselves without the computer and that they can stop when asked. The stopping when asked was already reinforced in them from the _creation_ of content. My son, unfortunately, is allowed by his mother to consume far too much content and he has not developed the limits that his older sisters have. And I'm having a very hard time getting him away from it, especially as she encourages it!
But sometimes I can persuade him with the hammer, some new nails, and some nice scraps of planks! He's already built a cage for the rabbit (with his father's help of course).