When to, and when not to pluralise month?

Question

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"

and

"Victor is 7 months old"

What is the grammar behind this?

Answer 1

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.

Answer 2

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.