Question

five-year=old和five years old用法有什么区别

Answer 1

five years old是名词，作定语修饰另一个名词 He was only five years old
five-year-old是形容词 并且year不能加s。 The five - year - old boy used foul words

Answer 2

five-year-old为合成形容词，一般要用连字符连接起来，而且，year也不用复数形式。又如：
an 80-page book 一本80页的书
a three-room apartment 三室的套房