16-year-old是形容词，比如说a 16-year-old boy,但是16 years old一般作表语，比如the boy is 16 years old.
形容词的用法
1.用作定语：This is a very interesting book.这是一本很有趣的书。
2.用作表语：This book is very interesting.这本书很有趣。
3.用作补语：We found this book very interesting.我们发现这本书很有趣。
4.“the+形容词”表示一类人：We should try to help the poor.我们应该设法帮助穷人。
5.“the+国家形容词”表示民族的整体：The Chinese are hardworking and friendly.中国人勤劳友好。
6.用作状语：Finally, he arrived here, cold and hungry.最终，他到这儿时又冷又饿。