How do hyphens modify the meaning of "n-month-old"?

I see three different ways of hyphenating the phrase "six month old".

Six-month old:

A six-month old poses with a machine gun owned by supporters of the Free Syrian Army.

Six-month-old:

Fraud alert closes account of six-month-old baby girl

Six month old:

Rebecca Judd, a model, TV personality and a new mother to Oscar (her six month old son with AFL star Chris Judd) is a self-confessed happy snapper.

Is there a significance to these differences?

Solution 1:

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.

Solution 2:

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).

Solution 3:

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.