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Hi there, why do some people put "21 years old or 21-year-old?" Can you tell me why and how to use it? Thank you

I am 21 years old.

I am 21-year-old.

He is 26 years old
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Comments  (Page 3) 
The first way in which you put it 'I am 21 years old' sounds as if it should be followed by something else like where they are from or something like that. The second way in which you put it 'I am 21-year-old' in my opinion should be written as I am a 21-year-old, and in comparison this way of putting it sounds stronger. I would say without the use of the '-' the sentence has a flow that doesn't break down.
AnonymousThe first way in which you put it 'I am 21 years old' sounds as if it should be followed by something else like where they are from or something like that.
I disagree.

This question was answered (and approved) eight years ago.
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 doriscornago's reply was promoted to an answer.