When to, and when not to pluralise month?

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?

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.