There is no doubt that the word year is a noun* in English. However, because of compound adjectives (the English, they love to compound! see also compound nouns), a lot of nouns can be made into adjectives**.
What are compound adjectives?
Compound adjectives are usually made up by more than one word grouped together using a hyphen. i.e. James is a well-known lawyer.
Let’s have a look at a particular kind of compound adjectives, those made by a number followed by other words. i.e. My sister has a three-year-old daughter.
If you’re reading this post with due care, you’ll notice that I’ve written a three-year-old daughter and not a three-year
s old daughter. Yep! three-year-old is a compound adjective and in English (unlike in languages like Italian or Spanish) adjectives can’t be made plural. Moreover, you need to put a hyphen both between three and year and year and old (think of three-year-old as a single word)
That being said, if I wanted to slightly change the above sentence into My sister’s daughter is three years old. As you’ve already guessed, I would have to add an ‘s’.
More examples of this type of compound adjectives: a fifteen-hour flight, a two million-dollar house, a two hundred-kilometre ride, a five-minute phone call, a four-door car.
In conclusion, the right answer is a six-year-old boy or you could even get rid of boy and be left with a six-year-old (which is actually a compound noun).
I hope this question won’t bug you anymore 🙂
*Nouns are words used to identify people, places, things, and ideas. i.e. A famous singer.
** Adjectives are words that refer to the qualities of people, things, or ideas, or which group them into classes. Most adjectives can be used with a noun and, in English, usually come immediately before it in the sentence. i.e A big city.