Which one of the following is correct?
1. It is 10 year old box
2. It is 10 years old box
3. She is 10 years old
4. She is 10 year old
Also please tell me the grammar behind it
It is a ten-year-old box. -- the compound adjective precedes the noun it modifies. This box is ten years old. -- the compound adjective goes after the linking verb as a complement and postmodifies the subject.