A woman in her late thirties posts a simple complaint online: “Men can have kids into their seventies. I’ve only got a few years left. That’s so unfair.” Within hours, her mentions are a war zone. Some men sneer about “geriatric eggs.” Others write manifestos about feminism and hypergamy. A few women defend her, but the thread collapses into the same argument we’ve been having for decades. In Britain, a thirty-four-year-old woman recently sued her ex-boyfriend, claiming he had “stolen her childbearing years.” After ten years together, he ended the relationship without fulfilling his promises of marriage and children. Now,...