(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] 就可以了不客气~