When the training wheels came off, probably.

But I guess you're talking as if nobody ever rode a bike until they got their first fixed gear in the last five years, so in that case:

On my way home from the co-op after swapping my Dicta freewheel with a no-name fixed sprocket. It was late in the winter and the roads were thinly covered in icy snow. Don't remember what mistake I made (no traffic), just that my hip hurt and I felt stupid for whatever it was. I don't remember it all that vividly because I'd already ate **** dozens upon dozens of times that winter, including earlier that night on the way TO the co-op to swap my Dicta freewheel with a no-name fixed sprocket. Didn't see a red light until too late, hit the brakes hard and wiped out.