(1) sixteen years old
(2) sixteen-year-old
(3) 16/sixteen
三个意思上都没有分别，但(1)(2)比较正式；(3)比较非正式，较常在日常生活中使用。
如果是介绍一个人，那就说:
(1) He is sixteen years old.
(2) He is sixteen-year-old.
(3) He is 16. / He is sixteen.
要说自己，把 [He is] 换成 [I am] 就可以了
追问太给力了，你的回答完美解决了我的问题！
追答不客气~