Why is the singular of "year" used in phrases like "72-year-old" and "20-year jail sentence"? [duplicate]

Possible Duplicate:
Pluralization rule for “five-year-old children”, “20 pound note”, “10 mile run”

72-year-old Giselle Gilbert was taken to hospital.
He was given a 20-year jail sentence.

Why is the singular of year used in these sentences?

Solution 1:

##-year-old, hyphenated is used as a pre-modifier (before the noun) while is ## years old is a post modifier.

The man is 21 years old.

a 21-year-old man

We could also use the singular hyphenated version in place of the noun:

My sister has a 10-year-old meaning that she has a child of 10.

That should clear up the usage for those who seemed unsure. Unfortunately, I am not quite sure of the reason for the two forms - I will look into it, though.

Hope that helps.