英语问题a fifteen-year-old与a fifteen-years-old等到底有什么区别呢?我一直都没搞懂,

英语问题a fifteen-year-old与a fifteen-years-old等到底有什么区别呢?我一直都没搞懂, fifteen-year-old 和fifteen years old 有什么区别?包括他们各自的用法 fifteen-year-olds和a fifteen-year-old boy fifteen-years-old的用法与区别 请帮我改下英语作文My Net Pal's name is Tony .He is a fifteen-year-old 请教一个英语问题?a 3 -year -old boy 和a 3 years old boy 有什么区别吗?还是这两个相 英语：变一变.She is fifteen years old.(对画线部分提问）画线部分：fifteen years he's a fifteen-year-old boy.[改为同义句 he’a boy 英语翻译Henry is a fifteen-year-old boy .He studies in a middle 有木有He is a fifteen-year-old American boy这样说 英语翻译Parent helps child.Westley Moore was a fifteen-year-old I am fifteen years old this__