We used to weld motorcycle bars to the stem back then. Can't remember what we called it but were riding the fire-breaks on something like a Raleigh Red Triumph with sturmey-archer 3-speed hub and 27" chrome rims (wet chrome and rim brakes were disasterous).
62 We used to call it "dirt tracking" in the late 1960's - most summer weekends were spent tooling around on the footpaths and bridleways around the South Downs on bikes that that, which broke all the time (yes, mainly handlebar stems), but we loved the adventure and the adrenaline and the fun. Moved to Australia 30 years ago and took up "mountain biking" again 12 years ago, and wondered why I ever stopped.
I built my own mid-drive MTB 10 years ago, just because I could, and because I knew one day eMTB's would be a thing - it was a bit noisy, the battery (in a backpack) lasted 5 km if I was lucky, and the bottom bracket freewheel shat itself every couple of weeks. Bought myself a Levo for Christmas because finally after test riding quite a lot of eMTB's over the last few years, I realised Specialized have got it sorted - so well, sometimes I forget I'm not riding my Mondraker (which I still ride half the time), and still loving it as much as I did when I was a kid.