usage - Difference between "two years old and two-year-old"

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?

Answer

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.