"X year old" is a phrase that can be either an adjective or a construct of the verb "to be" (I guess this technically makes it an adverb... sorta)Originally Posted by Penny
When used as an adjective the correct thing to say is "year old"
"You have an 8-year old child"
When used as a verb construct it must agree with the noun in terms of quantity.
"My child is 8 years old"
"My child is 1 year old"