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This is a general reference question. Check this definition of year in OAED, definition number 5 in particular will give you information about the usage of the word which will answer your question. – Irene Feb 24 '12 at 16:26
+1 for the AFL-related quote (and the decent question). – Mike G Mar 16 '12 at 4:57
up vote 4 down vote accepted

There is a convention in English that if multiple words work together as a single adjective -- usually an adjective/noun pair -- they are hyphenated for clarity. For exmample, if you have a plan that covers 5 years, you refer to it as a "five-year plan". This avoids any confusion that you might mean that you have five year-plans as opposed to one five-year plan. Similarly an "open-door policy" is a policy about open doors; an "open door policy" is an open policy about doors. An "old-dog leash" is a leash for old dogs; an "old dog leash" is an old leash for dogs. Etc.

Depending on the context it may be immediately obvious anyway, you may be able to figure it out with a second or two of thought, or it may be truly ambiguous. But in any case the convention helps make it immediately clear rather than burdening the reader.

In this case the adjective is made up of three words, "six month old". Technically if you wrote "six month-old babies" this would mean that you are referring to six babies who are each one-month old, while "six-month-old babies" would mean an unspecified number of babies who are each six months old. "Six-month old baby" doesn't make much sense. An old baby who is or has six months of something? I'm sure people would realize you meant "six-month-old baby" in this case, but other examples can be ambiguous, as I tried to explain above.

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Is there a difference in usage and meaning?

The meaning of "six month old" is the same in all of your examples (literally that the person in question is six months old).

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He's asking about the different hyphenation patterns. – slim Feb 24 '12 at 16:47
@slim but he's asking if there is a difference in meaning, there is not – kekekela Feb 24 '12 at 16:54
heh, well if you remove the question he's asking and replace it with your own that changes things of course – kekekela Feb 24 '12 at 16:55
@kekekela, I think slim's edit was correct: this is what the OP was meaning to ask. – Marthaª Feb 24 '12 at 16:59
No matter how you phrase it it seems like you are asking if there is a difference in meaning between the three different examples of usage. My answer was no, as is slim's more verbose version apparently. I'm not really sure what the issue is, other than replying with a yes/no makes less sense when the question is subsequently edited to a "how does" which played more directly into what slim wanted to respond with. – kekekela Feb 24 '12 at 17:06

The hyphenation is a matter of style. I think the first one ("six-month old") is questionable, but the others are both OK.

The phrase six-month-old is used in two senses here.

As an adjective phrase:

The six-month-old child smiled

As a noun phrase:

The six-month-old smiled

It's a common phrase, even though it leaves out the noun for the person/thing it's describing. It's common for a parent to say "I have a two-year-old", for example.

Using hyphens makes it clear to the reader that the phrase is standing in for a single word (a noun or an adjective). But the hyphens or not necessary, and using spaces does not change the meaning.

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It's probably best to leave out the hyphens.

You only need the hyphen if you want to say that the "six months" are some specific unit on their own.
So in a business you might have a specific four week accounting period that you refer to as a "four-week" then you would use the hyphen. But it's not needed to refer to any 4 weeks or 6 months.

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