我是一个十五岁的男孩用英语怎么说？

I am a 15-year-old boy./I am a boy of 15 years old.
当年龄作定语时，需要用数字-year-old的形式，如果用of结构，就可以直接用数字+ years+ old形式，在这里更正一下楼上的说法

1.I am a fifteen years old boy.
2.I am a fifteen-year-old boy.
这两种说法都行!

I am a 15-year-old boy

I am a 15 years old boy.

I am a 15-year-old boy