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!)