Question: what is the difference between years old and year old?
Answer: If using as the adjective of a noun (usually before the noun) it's “year old” as in “Ten-year-old car” if stating the age of the noun (usually after the noun) it's “years old” as in “the car is ten years old”. ... So “ The one year old car is one YEAR old”. If used as a pronoun for a thing such as a race horse.