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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?

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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.

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  • Fantastic, thanks for the explanation. I must not have been at school the day that was taught.
    – Terry
    Commented Jan 27, 2016 at 16:13
  • Unfortunately I don't think this approach to compound adjectives is a regular part of any curriculum, but I definitely think it should be!
    – John Clifford
    Commented Jan 27, 2016 at 16:14
  • It is a very simple method.
    – Terry
    Commented Jan 27, 2016 at 16:15
  • @Terry What's more likely is that your school never taught it. Government schools aren't known for being high quality.
    – ahnbizcad
    Commented Jan 27, 2016 at 22:19
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    It's also important to recognize that "My goddaughter is 13 months old", you do not hyphenate. Lots of people hypercorrect these and hyphenate at all times, because they don't recognize the purpose of the hyphens is to convert the phrase "13 months old" into a single word that can be used as an adjective to modify "goddaughter".
    – Monty Harder
    Commented Jan 27, 2016 at 22:52
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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.

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    I might add that pre-nominal modifiers never take plural endings, even if they are always plural. For instance, the correct phrase is shoe store, even though shoes always come in pairs -- *shoes store is simply wrong.
    – John Lawler
    Commented Apr 9, 2017 at 0:16
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    Never seems to me a bit strong; but maybe that's only because I took an arts degree and grew up in Alabama during the Civil Rights movement.
    – StoneyB on hiatus
    Commented Apr 9, 2017 at 2:00
  • There are different spelling variants used for 'nine days wonder', according to Wiktionary.
    – Edwin Ashworth
    Commented Aug 6, 2020 at 14:14
  • I assume nouned (deleted) usages retain the hyphens? 'He has the mind of an eleven-year-old.'
    – Edwin Ashworth
    Commented Aug 24, 2023 at 12:02
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Realistically, both options are acceptable - hyphens or no hyphens.

However, I opt for the hyphenated version because it has a clear meaning and makes it an adjective, which presumably is what it is intended to be.

See sumelic's link, as that has a very good explanation.

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