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What is the difference between 'two years old' and 'two-year-old'?

Are they the same or not?

What is the function of using dash in this phrase: 'two-year-old'?

When we use dash we cannot say 'years' why? What's the reason for that?

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    I don't think this question is "entirely answerable with a dictionary".
    – user230
    Dec 16, 2014 at 15:39
  • 4
    "I have a son, he is two years old." "I have a two-year-old son."
    – eyam
    Dec 16, 2014 at 15:40

3 Answers 3

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They are not quite the same, but they are similar.

Two-year-old is an adjective. You can say, two-year-old girl, or two-year-old cat, or two-year-old child. Sometimes, two-year-old is used as a noun on its own, and it that case ("My two-year-old", say), child is usually implied, although it could refer to an animal if the context is clear. It's a compound adjective, with dashes to make it clear it is all one phrase, and it is usually pronounced with the words run together. You might see it without the dashes, and it is usually clear what is meant.

You can't say "My daughter is two-year-old". In that case you say "My daughter is two years old".

I think the reason why it's year, when with dashes, is because the words are run together, and years would be hard to say.

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    "You can't say "My daughter is two-year-old"". Although it's worth noting some media including BBC news seem to have adopted the style of saying "My daughter is two-years-old" with hyphens. I hate this, but there it is.
    – Steve Jessop
    Dec 16, 2014 at 17:34
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    You can say "My daughter is a two-year-old", though. And the phrase can be written with or without the hyphens
    – Karen
    Dec 16, 2014 at 17:45
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    The reason is not that it's hard to say, it's because you don't pluralize adjectives, even compound adjectives. For example: two car garage (not "two cars garage"), three family house.
    – Ruan Mendes
    Dec 16, 2014 at 19:02
  • I agree with @Karen. I most often hear "a two-year-old"--as a noun.
    – Richard
    Dec 16, 2014 at 20:42
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Some adjectives can only be used to modify nouns, for example the adjective indoor. We can talk about:

  • indoor swimming pools

But we don't usually say:

  • *The pool was indoor (not good)

We call adjectives that appear before nouns attributive adjectives. The adjective indoor is called an attributive only adjective.

Other adjectives can't usually be used before a noun. We usually find these adjectives as the complements of verbs like BE, FEEL or BECOME. So we can say

  • She was afraid
  • She felt afraid

But we cannot say:

  • *an afraid girl (not good)

Adjectives that we use like this are called predicative adjectives. The adjective afraid is a predicative only adjective.

We can use most adjectives as attributive adjective and predicative adjectives:

  • a huge elephant
  • The elephant was huge.

Sometimes we have two adjectives that look similar and mean the same thing. One of them is attributive only, and the other predicative only. For example, the adjectives live and alive. When these words are used to describe things that aren't dead, we use live as an attributive adjective and alive as a predicative adjective:

  • a live snake
  • The snake was alive.
  • *an alive snake (wrong)
  • *The snake was live. (wrong)

The term two-year old is used as an attributive only adjective phrase:

  • a two year old whisky
  • *The whisky was two year old (wrong).

The term two years old is used as a predicative only adjective phrase:

  • *a two years old whisky (wrong)
  • The whisky was two years old.

Note

We can also use the term two year old as a nominal phrase. We can use it like a noun.:

  • I have two children: a two year old and a three year old.

Notice that we use the attributive adjective here because we mean: a two year old child.

Hope this is helpful!

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    Huddleston & Pullum seem to say that two-year-old is a compound adjective (a single word), while two years old is an adjective phrase with a pre-head NP modifier. (The Cambridge Grammar of the English Language p.551-2 and 1660)
    – user230
    Dec 17, 2014 at 14:12
  • @snailboat +1 Quite right! :) I was going to put that in - but thought it might be too complicated for the task at hand. Can kind of get round it using Adjective Phrase because compound adjectives (all adjectives) are also Adjective Phrases ;)
    – Araucaria - Not here any more.
    Dec 17, 2014 at 14:54
  • "live" can be used as a predicative adjective when it is used metaphorically: "*the circuit was live when I checked it" or "the program is live right now"
    – David Siegel
    Jun 8, 2022 at 5:45
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While "two-year-old" and "two years old" are similar, there is a distinct difference beyond the "s" in that the first is an identifying number or categorizing using numbers and the other is about ordering or measuring.

The distinction is similar to being numbered as a runner versus placing in a race. The placing of runners as they finish a race is an order, like "first" and "second" compared to simply the identifying of runners using numbers. "There were 12 runners and my shirt had number 2, but I was number 2 in the race because I finished in second place!"

Usually people will refer to their "two-year-old" if they have more than one child, as a means of identifying the child rather than simply stating the child's age. It then indicates the "category" of the child, not as much the age.

There are similar phrases, such as "To work, the nail must be two inches long" (measure) vs "Please hand me that two-inch-long nail" (identifier). Or "My son has grown, he must be five feet tall by now" (measure) vs "look at that five-foot-tall person" (identifier). In each of these, the first example counts the amount or is a measure (like runners finishing a race), the second example it is about identifiers/categories (like the numbers on a runner's shirt).

Hyphens are optional.

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