About nine years old, at one of the local lakes with my family.
The swimming area was marked off with a bunch of logs whose ends clipped together with metal snaps. There were also a couple of those boundary logs that were floating free in the swimming area--good for playing on.
My two sisters and I went to one of the logs, and as I was climbing up on one end they pushed it a bit from their end, and I felt something dig into my leg. Didn't really hurt, though.
I got back off the log and felt my leg. It felt kind of funny--not smooth, but sort of lumpy in a way. So I walked out of the water, back toward where our blanket was. I was still not sure what was going on but my fingers told me that the leg wasn't right.
Got to the blanket, pulled up the leg of my swim trunks a bit, and saw that the metal clip on that log had gone into my thigh and torn up the fat on it (not that there was much--I was a skinny runner-to-be).
My mom, who was an RN, assessed the situation and figured I'd have to get a tetanus shot and be sewn up. She went to the lifeguard shack to see whether they had any medical stuff--nah, nothing but bandaids. So we went to the car, where she kept a little first aid kit in the glove compartment, and she wrapped an elastic bandage around my thigh. Dad drove us all from the lake to the base hospital. He drove pretty fast, but this was out West where everything was spread out, so it took maybe an hour to get there.
They got me sewn up fine. I may have bled some, but probably not much. It was some years later, and I had more anatomical knowledge, before it hit me that the wound had been about an inch from the femoral artery (and not much farther from the testicles). If the artery had been clipped I most likely would have bled out.