a usage of noun
a seven-years child ,if this phrase is correct, then ,He is seven-year old. And
he is seven-years old.these two sentences are correct,right?

Question

a usage of noun
a seven-years child ,if this phrase is correct, then ,He is seven-year old.  And
he is seven-years old.these two sentences are correct,right?

Answer

Correct sentences:

He is a seven-year-old.
He is a seven-year-old child
He is seven years old.

Notice that we add hyphens and remove the "s" when we treat the phrase as an adjective (or, an adjective which represents a noun).