grammatical number - "a ten years old boy" or "a ten year old boy"

1) a ten years old boy is sitting on the couch.

2) a ten year old boy is sitting on the couch.

3) a ten-years old boy is sitting on the couch.

4) a ten-year old boy is sitting on the couch.

Which is correct?

Answer

Generally, when units of measure are used as adjectives, or as part of a compound noun, they are singular. When expressed as simple nouns, they are plural. Thus,

A ten-year-old boy is sitting on the couch.

The boy sitting on the couch is ten years old.

The boy sitting on the couch is a ten-year-old.

As for the hyphenation, exact usage is a matter of style, but the hyphen will determine how to interpret the phrase.

Ten year-old trees

refers to ten trees which are each one year old, whereas

Ten-year-old trees

refers to some trees which are each ten years old.