Mary smiled the way her mother did when she was at Mary's age.

Smile sth 是因为某事情发笑
smiled the way...中 the way是 smile的宾语， her mother did 是the way 的定语 即 她母亲年轻时候的举止——“the way her mother did when she was at Mary's age”

然后说说 as if： as if 是“好像”的意思，放入此句语境中不妥。