when to remove the plural s [duplicate]

Question

Since I was first learned English, I know that any number greater than 1 comes with s for plural (except for some exceptions like Mice and Feet). 6 Cars, 2 Trees, 3 Dogs, 9 Years

So if someone ask me how old are you I will say 6 years old

but when I read a report in news website I find they say a 6 year old boy (not years)

so when to remove the plural s?

Answer 1

You remove the -s when it comes to a compound hyphenated set of adjectives modifying a noun:

You are a 6-year-old boy. (6-year-old acts as an adjective modifying the noun "boy")

Or you can also remove it when it's a compound hyphenated noun:

A 6-year-old is singing (6-year-old is not modifying a noun, because it's standing as a noun by itself, it's a noun)

Since it's a noun, it can take plural form:

6-year-olds are playing in the rain.

If it's not hyphenated, "6 year old" is incorrect.