The No. 1 movie at the box office over the weekend was ... "The 40 Year-Old Virgin." And, no, that's not a mistake; almost all of the movie's promotional material (including its official Web site) leaves out the hyphen between "40" and "year."
E&P has a story on the matter, including a rundown on which news outlets are adding the hyphen, which are leaving it as is, and which are inconsistent.
It's an interesting problem. I think most copy desks would add the hyphen "to avoid confusion," although I doubt there's a soul out there who thinks the film is about 40 abstaining babies.
On the other hand, as one reporter points out in the article, there is at least one promotional image (used by IMDb.com and Rotten Tomatoes) that includes the missing hyphen.
My vote? Go without the hyphen. There's no real confusion, and that's what the official site does. If you can't use that as the deciding factor, I'm not sure what you can use.
And note that this might seem to contradict my view that movie titles should be capitalized regardless of what the movie makers say. I stand by that; proper nouns are capped in our language, and the lack of them is more likely to cause confusion than the lack of a hyphen. And capitalization is more a logo issue than a language issue; I can't say the same about hyphenation. (I could be persuaded, though, if it were proved that the producers use the hyphen in writing about the movie but intentionally didn't use it in the logo. I can't figure out why that would be the case, but it's possible.)
It's a fine line, I know, but that's where I choose to draw it.
(Also, check out Romenesko's Letters section. Some readers are chiming in.)