I learned to type at around 4 by writing letters on a type writer and asking my parents how to spell word by word until I remembered them. I must have driven them crazy, especially as this was a noise mechanical typewriter...

I then learned BASIC on a VIC-20 at 5 by copying my dad and experimenting (at one point I spent an afternoon after I learned about POKE, POKE'ing through large chunks of memory and being delighted every time I managed to provoke weird sounds or visuals).

Many developers I've known have similar stories.

Looking at my own son (3 now) I absolutely think starting at the command line makes learning some basics like that fairly reasonable to achieve for most 4-5 year olds at least. A mouse is hard - it requires a lot of coordination. Touch interfaces works, but limit how far you can easily go in some directions (though they work great for others).