Can someone please explain the correct way of pluralising month, for instance why do we sometimes use months or month, such as:

"Victor is a 7 month old baby"


"Victor is 7 months old"

What is the grammar behind this?

  1. '7 month old' is actually '7-month-old'. It serves as an attribute that modifies a noun. (e.g. old in the old dog.)

    It is adjectival and therefore has no plural form. You need to put an article in front of it and a noun after it.

    For example: He is a 7-year-old boy.

  2. 'is 7 months old' is a predicate that states something about the subject. (e.g. old in the dog is old.)

    In this case, adding -s to the end of year is the proper and regular way.

    For example: The boy is 7 years old.

| improve this answer | |

Victor is a 7 month old baby.

In that case, 'a' acts as a determiner before the term '7 month'.

But in this one, '7 months' itself acts as a determiner.

Victor is 7 months old.

| improve this answer | |
  • 1
    No, the predicate is not a determiner in your second example. – tchrist Oct 7 '16 at 13:20

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