Which is right, two-day-old Tom or two day old Tom?

Since the phrase comes before “Tom”, it’s a compound modifier – which we generally hyphenate. If they were after “Tom”, no hyphens would be used: “two-day-old Tom” or “Tom is two days old”.

Two-day-old Tom is right

A good clue to determine whether you should hyphenate the year or day old phrase is to see if a noun comes after it. If there is a noun, hyphenate:
two-year-old toy
sixty-year-old woman

If the sentence is simply stating that someone or something is so many years old, then don’t use a hyphen:

Her Mom turned eighty years old today.

Thank you. Both answer help explain why the hyphen was used. I asked some of my classmates and they said it looked/sounded off, thought I’d get a second opinion.

@kaywizard :You’re welcome!

We prefer “grammar goblins” to “grammar Nazis” round these parts.

I think of the grammarians as the “Grammar Stewards,” and I appreciate you immensely!

Also, if you are referring to a cat, “tom” should not be capitalized. ;)

Sorry, this question goes out to the grammar goblins and stewards.