Both “years old” and “year-old” are correct, but they are used differently in a sentence.
1. Use “years old” after a linking verb (like “is” “am” are”).
Examples:
- She is twenty years old.
- My cat is two years old.
Here, “years old” acts as a phrase describing age. It’s plural because you’re talking about multiple years.
2. Use “year-old” before a noun (with hyphens).
Examples:
- She is a twenty-year-old girl.
- I have a two-year-old cat.
Here, “year-old” is a compound adjective modifying the noun. The hyphens are necessary because they connect multiple words to function as a single description. Without them, the sentence could be unclear.
Why Use Hyphens?
Hyphens help avoid confusion by showing that the words work together as one unit.
Compare:
- I met a ten-year-old boy. (Correct: “ten-year-old” acts as one adjective.)
- I met a ten year old boy. (Incorrect! Without hyphens, “ten” and “year” could be separate, and old boy can be possibly referring to an old boy, not necessarily his age).
Common Mistakes to Avoid:
Incorrect: He is a five years old child. (Should be “five-year-old child.”)
Incorrect: My sister is ten-years-old. (No hyphens are needed: “ten years old.”)
Incorrect: I have a three years old puppy. (Should be “three-year-old puppy.”)