Question

she is eight years old为什么不是she is eight-year-old

Answer 1

she is eight years old--------因为eight years old只能做表语，后面不加名词。

但是 ：she is an eight-year-old girl 就对了，因为eight-year-old（做定语）后面必须加个名词，这里是可数名词单数。

Answer 2

she is eight years old

eight-year-old 中间加有连字符就变成了形容词
所以我们可以把句子改为
she is an eight-year-old girl

Answer 3

She is eight years old ,或者也可以she is an eight-year-girl