What's the difference between year old and years old?

Should I say: I'm X year old or I'm X years old?

Maybe are both correct?

Looking forward to your response.

Still expecting to an answer...

one year old

two years old


twenty years old


fifty years old


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Thank you, CJ.

I see many people write I'm twenty-year-old (not years old, although it is plural). That's why I asked my question.

Could you explain this please?

Did you understand my question, CJ?
As a modifier, use the singular.

A four-year-old boy.

A 20-year-old whiskey.

To simply say "I'm 20 years old" then use the plural (except for 1).

She's 20. She's 20 years old. She's a 20-year-old woman.

He's one. He's one year old.

When referring to children, we do sometimes say " I have a four-year-old and a nine-year-old." In those cases, they are the nouns and are preceded by an article. I find it rare to use this with adults.
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You don't use plural forms when the number is part of a compound noun or adjective.


A centenarian is a 100-year-old person. (this sounds a bit odd, but should be alright grammatically, and is only for demonstration purposes :-)

A centenarian is a person who is 100 years old.

As a noun:

The thirteen-year-old who lives on the other side of the road keeps staring at me.
Thanks Grammar Geek and Noctivagus. I got the idea!Emotion: smile