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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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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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    They 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. – Colin Fine Feb 23 '14 at 13:13

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