其实句中的 36-year-old 并非后置定语，而是表语，换句话说，这里的 36-year-old 不是形容词，而是名词。比较：

▲用作形容词的用法：

She buried her sixty-year-old husband. 她丈夫去世了，享年60岁。

An 11-year-old boy committed a burglary. 一个11岁的小男孩犯了入室盗窃罪。

Her two-year-old son toddled into the room. 她的两岁的儿子摇摇摆摆地走进屋里。

One call came from a mother concerned for her thirteen-year-old daughter. 一位挂念着自己13岁女儿的母亲打来了一个电话。

▲用作名词的用法（可数）：

She is very responsible for a six-year-old. 对于六岁的孩子来说，她算是很靠得住的。

He was not considered competent to teach seven-year-olds. 他被认为没有能力教7岁的孩子。

Tim has an average level of vocabulary for a 3-year-old. 蒂姆掌握了相当于3岁孩子的平均词汇量。

We have two girls, a six-year-old and an eight-year-old. 我们有两个女儿，一个六岁，一个八岁。

I’m still amazed that a terrified 19-year-old would have the presence of mind to reason with her kidnapper. 一个受惊的19岁少女能够沉住气与绑架她的人讲道理，这仍然让我感到惊奇。

▲在 Xandra is an athletic 36-year-old with a 21-year-old’s body. 中，36-year-old 是名词，用作表语，其前的 athletic 是形容词，意思是“体格健壮的”，用作定语修饰名词 36-year-old；其后的介词短语 with a 21-year-old’s body 也是修饰 36-year-old 的定语。句子的字面意思是：汉德拉36岁，身体健壮，具有21岁小伙子的身体。稍加整理即为你给出的翻译：36岁的汉德拉身体健壮，有着21岁小伙子的体能。