Difference between ‘years old’ and ‘year-old’

Toronto’s Old City Hall

When to use ‘years old’?

When stating the age of a person or thing as the subject of a sentence, which means that the person or thing comes first in the sentence, you should use the phrase ‘years old’ along with the verb ‘to be’, for example:

Sue is eight years old.
Buckingham Palace is over 300 years old.
My sister is forty years old.
The Eiffel Tower in Paris is over 130 years old.
London is at least 2,000 years old.
My car is seven years old.
His dad is 45 years old.
Ronaldo began his professional career when he was 15 years old.
Montreal is hundreds of years old.

The only exception is one year old – it’s always one year old, never one years old, for example:

My baby is one year old.

We could also say am / are / is + (age) without ‘years old’:

She’s eight.
My brother is twenty.
My kids are three and four.
My baby is one.

When to use ‘year-old’?

When the age is being used as an adjective before the name of the person or thing, then we say year-old and not years old. Moreover, you need to put a hyphen between the words. For example, you would write the ‘six-year-old child’. ‘Six-year-old’ is an adjective that describes the child. Examples:

Ann is a 15-year-old girl.
A 10-year-old Pole has won the 1st prize in an international Mandarin language competition.
A 200-year-old bottle of Champagne was recovered from a Baltic shipwreck.
A 23-year-old swimmer swam across the French Channel.
A 92-year-old woman finished San Diego marathon.
My six-year-old daughter is learning English now.
My special guest was an eight-year-old girl today.
Police confirmed that a 15-year-old boy was driving the car.
What is the average life expectancy for a 65-year-old man and woman?
This is a 500-year-old sword.
This is the 800-year-old Notre-Dame Cathedral.

Sometimes age is used as a noun. Year-old is also hyphenated in these sentences, for example:

16-year-old is accused of leading police on a chase in a stolen car.
A 17-year-old was arrested and accused of stealing van.
Eight-year-old won the European Rapid and Blitz Chess Championship.
For being a 70-year-old, he’s still very strong.

Photo: Toronto’s Old City Hall