90　at the age of和by the age of

① at the age of意为“在……岁时”，表示一个时间点。

He began to study English at the age of three. 他在3岁时就开始学英语了。

She went abroad with her father at the age of ten. 她10岁时同父亲一起去国外了。

② by the age of意为“在……岁之前”，表示截止到某一时间之前，如果by the age of在句中指过去时间，谓语动词要用过去完成时。

He had read many novels by the age of fifteen. 在15岁前他就读过许多小说了。

By the age of ten，she had learned to play the violin. 在10岁前，她就学会拉小提琴了。