English Grammar Gotcha: “year old” versus “years old”
In English grammar, in general the plural (more than one) form of a noun is formed by adding “s” at the end of the noun. For example: dog (singular), dogs (plural), cat (singular), cats (plural), car (singular), cars (plural), etc.
There are many special case exceptions to the adding “s” rule: mouse (singular), mice (plural), goose (singular), geese (plural), sheep (singular), sheep (plural), etc.
There is subtlety with using units of measure as an adjective. When units of measure are used as adjectives, or as part of a compound noun, they are singular. When expressed as simple nouns, they are plural.
For example, these are correct English:
Bob is an eight year old.
Bob is eight years old.
My eight year old’s name is Bob.
The Bay to Breakers is a 7.46 mile race.
The Bay to Breakers race is 7.46 miles long.
More at: https://ell.stackexchange.com/questions/6434/a-ten-years-old-boy-or-a-ten-year-old-boy
(C) 2018 by John F. McGowan, Ph.D.