13 Month Old or 13-Month-Old? [duplicate]

I have just installed Grammarly and it showed up something which i am not sure of.

It corrected '13 month old' to '13-month-old'.

The context is

I ask because my 13-month-old God daughter seemed like she was a little resentful towards another child who was visiting at the same time.

Which would be correct, and why?

Solution 1:

General rule of thumb with compound adjectives like this is to hyphenate them if they are made up of words which modify the base adjective and couldn't be used independently. The best way to gauge this is to write the sentence out with each word making up the adjective individually and see if they all still make sense:

My 13 God daughter

Obviously doesn't make sense

My month God daughter

As above

My old God daughter

Makes sense, but doesn't convey the meaning you intended.

"old" here is the base adjective; 13 and month are modifying it, so you hyphenate. 13-month-old.

Solution 2:

The basic mnemonic rule, which is called the Eleven-Year-Old Boy Rule, is

• one-word modifiers precede the noun they modify
• modifiers of more than one word follow the noun they modify.

One way of making a modifier of more than one word into a one-word modifier is to hyphenate it.
Like an eleven-year-old boy versus a boy eleven years old. Of course, that's just in writing.
In speech hyphens are inaudible so we just use the word order.