I'm guessing it was my 11th birthday, but who knows, I could be off by a year or two.  Somewhere there're undoubtedly medical records that could be found if it matters, but it really doesn't.  Dad was working on fencing some of the pasture in and he was about 3/4 mile from the house (around a corner and up a hill) and Mom was back at home when...

Wait, I've got to back up a bit.  I got the speedometer and was really really excited about it.  I think initially Dad helped me to set it up on the bike, but what I do remember is that I was fiddling with the front wheel quite a bit (because there's obviously something down there that needs to sense the speed of the wheel).   Anyhow, after taking the front wheel off and on several times for readjustments, I decided that the tightening of the lugnuts was really superfluous.   First of all, it was really hard to get them loose without stripping the corners off with the crescent wrench (i was capable of barely using most tools at that age, and didn't really do anything with any sort of efficiency).  Second of all, the weight of me being on the bike would obviously keep the wheel on.  What could go wrong?

So I rode out the driveway after another adjustment (which had gone quite quickly due to my already loose lugnuts) and lo and behold the speedometer was finally working.  So I pedalled off between the field and the river and got up somewhere between 10 and 15 miles an hour.  This was great, so now to see how fast I could go (what else is a speedometer for?).   I got to the end of the long field and started up the hill (losing site of the house and barn several hundred yards away across the field).  I'm sure I must have crossed the really bumpy part at the bottom of the hill where the culvert wasn't quite sufficient for the amount of rainwater that sometimes came down the hill.  But at this moment, I guess I was just concentrating on keeping up as much speed as possible and pedalling hard (as it was at the bottom of a pretty steep hill).  At another point in my childhood (I think later?) I obtained quite a collection of hubcaps over the years by checking the corner at the bottom of the hill.   After huffing and puffing I remember getting up the hill far enough to be aware of where my Dad was working on putting up barbed wire fence, but I don't remember if I went over and said hi.   But I did turn around and head down the hill.  It was really exhilarating.  I looked down and saw the magic number 20 mph on the speedometer and was enjoying the wind in my hair so much more than the other times I'd come down the hill pre-speedometer (it was the knowledge of the quantification of the speed that made it somehow so much better), but that feeling of triumph was quite short lived.   Within seconds of glancing down and seeing the needle at 20mph I had bounced through the rut in the  pavement over the culvert and of course bounced right off my front wheel.  I have a very vivid memory of looking down to see the front forks of the bicycle hit the pavement and stick, catapulting me headfirst over/through the handlebars so fast that I doubt I even had time to let go of the handle bars, much less put my hands up.  I'm pretty sure I didn't lose consciousness after landing on my face at 20mph, but it's possible I was concussed,  because I immediately sat up with an absolutely clear belief that the one sure sign of a cracked skull was bleeding.  I remember such great relief in those first few seconds after watching the wheel to continue to roll down the hill and off the road below that there was no blood (because of course I couldn't see my own face), but it didn't take long for enough of the wounds I'd opened up to bleed enough to drip off my chin or nose.  The first drop didn't quite penetrate my shock, I was in denial at that point and unwilling to believe I had a broken skull.  But after the flow continued without slowing up, all of a sudden I really thought my life was over and my brains were about to spill out of the crack in my skull.  

So I screamed.  And I don't mean just yelled a little, I erupted.  Some part of my brain still hadn't leaked out obviously (in hindsight) because I was savvy enough to run *up* the hill towards my Dad (who was a little closer than home and Mom).   I remember screaming incoherently to him when he came running to meet me about my cracked skull and my brains leaking out and not making really any sense at all (being quite winded from trying to run up the hill and the shock, etc...)

I remember the house being far enough away to be mostly out of hearing range, but I also remember screaming really really loud (well, you know, the whole cracked skull think was pretty traumatic).  Mom's memory of this event is that she didn't consciously hear me scream, but that she *knew* something had happened.    So she wasn't at all surprised when Dad came barreling in with me sobbing and bloody in the pickup truck.  Off we went to the doctor, who ended up sewing up three separate cuts, one over my eyelid, one on my forehead and one on my upper lip, but he really couldn't do much for the major road rash that covered the rest of my face.  And no, my skull wasn't cracked.  But the whole side of my face turned into one giant scab and I really must have looked dreadful (I remember only looking in a mirror a couple times, and I absolutely wouldn't let my parents take a picture of me (though they begged that I'd want to remember it later)).   Eventually of course it all healed up, and it's tough to find any of those three scars at this point, but it did end up being one of those rather valuable life lessons.  Blindly making assumptions to fit convenience can be really dangerous.