[Grammar] 40 year old

There is movie named "40 year old virgin"
just wondering why it is not "40 yearS old virgin" ?
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do not we say "yearS", when somebody ask us "how old are you?" and we answer ie : 36 yearS old ?

This film is more correctly spelt The 40-Year-Old Virgin (the capitals are used because it's a title).

We say 'I'm 40 years old' or 'I'm a 40-year-old man'.

Rover

Rover_KE said:

This film is more correctly spelt The 40-Year-Old Virgin (the capitals are used because it's a title).

We say 'I'm 40 years old' or 'I'm a 40-year-old man'.

Rover

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But writing 'year' without an 's' and 'a' in these sentences be wrong?
1) I'm a 40 year old
2) I'm 40 year old.

atchan said:

But writing 'year' without an 's' and 'a' in these sentences be wrong?
1) I'm a 40 year old
2) I'm 40 year old.

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1) You could say 'I'm a 40-year-old (man)', but you'd need the hyphens.
2) 'I'm 40 years old.'

Rover

When used with numbers, the measurements are in the singular before the noun.

A three-mile walk. A walk of three miles.
A 10-meter diving board. A diving board of 10 meters.
A two-week vacation. A vacation of two weeks.