Which one is correct?
- A six-year-old boy
- A six-years-old boy
I see both usage when I search in google.
I would say the first one is correct. We say:
The boy is six years old, but He is a six-year-old boy.
The snake is five meters long, but It is a five-meter-long snake.
The pool is twenty feet wide, but It is a twenty-foot-wide pool.
You need to drop the plural to form the adjective.
This page has more information and examples on the same.