Well, we may have answered the age-old question of what happens when you take your child to fencing with you almost every Saturday from the age of... around 7 weeks or there-abouts.
This recent development has been worked on over the last couple of weeks.

We returned home one Saturday and he amused himself for ages picking up a stick, "enguarde, fence" doing footwork down the hall, lunging at a cardboard box with a stick, then "push the button" on a mark on the box before returning up the hall to put his stick down "all done fencing". Put this in infinite loop. I believe he was imitating the fencers turning the box off or changing the weapon settings on the scoring box.
Next week he was dumping the pegs out of the basket and wearing it as a fencing mask, then using a peg as his sword. (Apparently it's a "fencing peg"). Put the mask on, pick up the peg, "Enguarde, ready, fence", a few steps to the glass door which then either got stabbed with the peg, had bub run into it, or both. "Halt", return to starting position, put mask and peg back down. "All done fencing", repeat on infinite loop.
Today at fencing he got hold of dad's foil and wanted to stab the walls with it. So I redirected him to the soft padded wall we use for target practice for the sake of the foil.
He'd been doing rather well with it, so when dad reclaimed the foil I softened the blow by offering to get him his own sword. Bub toddled with me to the weapon bag, I handed over a size 0 foil, stuck a target on the wall for him to aim for... and he was off.
Honestly, this is all imitation and being allowed to play. I'm not some pushy fencing mum giving him lessons, I swear! I'm a little worried he'll burn out and hate fencing before he turns 5 at this rate.
On the other hand, the only thing stopping him from having actual fencing lessons now would seem to be the ability to wear a mask. He's been trying them on, but they're still too big and heavy. (I've told him he can have a lesson and learn to fence with real swords when the size 5 glove and XS mask in the cupboard fit him. It's starting to seem disturbingly close... Until then he's supposed to be on the foam swords, although I'm not sure that's going to stick.)
There are lighter plastic masks, but they're huge and I've yet to find any kid that prefers them to the real thing.
And he seems to be developing a pretty good sense of distance...
You can see a slightly longer video here:
https://www.youtube.com/watch?v=IdOEDmEPEKI&feature=youtu.be