Deenie is about a thirteen-year-old girl.中的year为什么不用加s?

Deenie is about a thirteen-year-old girl.中的year为什么不用加s? I am a 11-year-old girl.为什么girl后面不加s Ma Yanli is a twelve-year-old girl.为什么要加- is a （）girl.答案是13-year-old为什么eight-year-old不对 She is a seven year old girl he is 18 years old.为什么year要加S a thirteen-year-old boy=什么? a thirteen-year-old boy的中文翻译 英语翻译She is a 8-year-old girl还是She is an 8-year-old girl. a girl of 后面加eight years old还是eight-year-old?为什么,麻烦详细一点 lucy is a（）girl.我认为eight-year-old与13-year-old都对,为什么答案选择后者 8.Kate is an eight-years-old boy.这里为什么year还要加s,不是有横线就不用加吗?还有