They are all 10 years old.
They are all ten-year-olds
What is the difference between these?
And, what is the reason why we must add s to the following?"
They are all ten-year-olds
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possible duplicate of Pluralization rule for "five-year-old children", "20 pound note", "10 mile run". Also “40-50 years old” vs. “40-50 year olds” when referring to a group, Ways to write “2000 year old”, Why is the singular of “year” used in phrases like “72-year-old”, etc., etc., etc., etc., ...– FumbleFingersCommented Feb 23, 2014 at 14:59
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The first two Google hits for "ten-year-old" show the adjectival and nounal usages respectively. And there are previous posts on hyphenated pre-modifiers and their unhyphenated predicative counterparts.– Edwin AshworthCommented Feb 23, 2014 at 15:48
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1 Answer
In "The are all 10 year old", '10 year old' is used to describe 'they'. In "They are all 10-year-olds", '10-year-olds' is a noun. The first is like saying 'He is a boy' and the second is like saying 'he is brainy'.
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1They are all 10 year old is not grammatical (in most varieties of English). Ten year old may be used as an adjective, but only attributively ("a ten year old boy"). And your last sentence is back to front: ten years old is an adjectival phrase like brainy, while ten year olds is a noun phrase like a boy. Commented Feb 23, 2014 at 13:13