I LEARNED TO ride my bicycle at the edge of a small German village. My parents fastened a set of training wheels to it, strapped a helmet onto my head, and gave me a gentle push down the road. With practice, the training wheels came off, and the helmet disappeared at some point during high school. It seemed like a natural progression in which “going helmet-free” was merely one of the rites that mark the gradual transition toward adolescence: the first drink, the first kiss, the first unprotected bike ride—although not necessarily in that order. I can’t remember being scolded by my parents for tossing the helmet, and...