When he was fourteen, he began to work. /At the age of fourteen.
句意:查理斯 狄更斯什么时候开始工作?
根据第三段第1-2行:Only until his father was out of prison could Charles go to school. At that time he was already twelve. But he did not finish school. Two years later, he began to work in a factory.
(查尔斯只有在父亲出狱后才能去上学。那时他已经12岁了。但是他没有完成学业。两年后,他开始在一家工厂工作。)
可知,他14岁开始工作。故填When he was fourteen, he began to work. /At the age of fourteen.