An 18 year old ...

[Odessa Dawn]

An 18 year old steals his father's handgun, tries to commit suicide along with his pregnant girlfriend, opens fire at police officers who arrive at the scene to save them and is killed when they return fire.
More: Guns and abortions - Israel Opinion, Ynetnews

Shouldn't we hyphenate an 18 year old? Is it an adjective phrase & as a result it doesn't need to be hyphenated?

Examples:

An eighteen years old = I think this one is correct. If I am not mistaken, it is an adjective phrase.
An eighteen-year-old = deadly wrong
A thirteen-year-old boy= right
A thirteen years old boy = wrong
I am thirty five years old. = The underlined part is adjective phrase, isn't it? As a result, we don't need hyphen.

Correct me pleas!

[emsr2d2]

An 18 year old steals his father's handgun, tries to commit suicide along with his pregnant girlfriend, opens fire at police officers who arrive at the scene to save them and is killed when they return fire.
More: Guns and abortions - Israel Opinion, Ynetnews

Shouldn't we hyphenate an 18 year old? Is it an adjective phrase & as a result it doesn't need to be hyphenated?

Examples:

An eighteen years old = I think this one is correct. If I am not mistaken, it is an adjective phrase.
An eighteen-year-old = deadly wrong
A thirteen-year-old boy= right
A thirteen years old boy = wrong
I am thirty five years old. = The underlined part is adjective phrase, isn't it? As a result, we don't need hyphen.

Correct me pleas!

I would hyphenate it. "An eighteen-year-old steals his father's handgun ..."
I'm not sure why you suggested that it should be hyphenated and then went on to say that "An eighteen-year-old" is "deadly wrong". On a side note, I don't really think you can describe any construction as "deadly wrong"!

[Odessa Dawn]

When the age is an adjective that comes before the noun and modifies the noun, or when the age is a noun, hyphenate. When the age is part of an adjective phrase after the noun, don't hyphenate.

More: Grammar Girl : Are You Using Hyphens Correctly? :: Quick and Dirty Tips ™

Your take, please.

[Barb_D]

When the age is an adjective that comes before the noun and modifies the noun, or when the age is a noun, hyphenate. When the age is part of an adjective phrase after the noun, don't hyphenate.

More: Grammar Girl : Are You Using Hyphens Correctly? :: Quick and Dirty Tips ™

Your take, please.

The age was the noun.