Fiveteen is a term used to describe girls that are attractive and sexually mature (at least in physical terms) but just a little bit too young to get away with. This can also be known as jailbait. See, jailbait.
Being male in the prescence of girls who are fiveteen has been known to cause intense mental anguish, as the mind is torn apart but competing feelings of lust and guilt. Fiveteen may or may not be related to the age fifteen - this is a topic of much debate over the centuries. 
You usually feel somewhat guilty when lusting after these girls, like you're a lecherous perv, but when they wear a v-neck shirt and lean over so often, they sort of want you to take a peek. And did you see the shelf ass on that tall girl in the denim skirt? Goddamn, she must be on the track team...
This changes 15 to be pronounced in the format of its fellow numbers 14, 16-19. 10-13 are oddities but at least they keep to themselves.
What are they like?
Girls who are Fiveteen are easily identified by their hyperactivity, perky breasts, and ability to find every single thing you say, like, totally hilarious. They are generally found in groups of 3 - 20, giggling and pointing out "totally hot guys", which happens to be anyone but you.
They are also often found on social-networking sites such as Myspace, where they will attempt to lure older, "totally hot" guys into having sex/going out with them. This is the girl of Fiveteen's primary way of seeking approval from her friends, as an older boyfriend shows that she is both hot AND cool. Recent studies have suggested a direct correlation between the girl's bra-size, and the oldest guy she can get to go out with her. Scientists say such a result is "not really that surprising, really."
How to get away with it
An easy way to justify copulating with a girl who is fiveteen is to deliberately misquote Chesnick's Rule, which states that the youngest age girl you can get with without fear of persecution by your peers, Ag, is related to your age, Ay, by the following formula:
(Chesnick himself was unsure whether to round up or down though, in the case that your age is an odd number - choose wisely)
However, this of course assumes your friends do not already know Chesnick's Rule, which in all likelihood they do as they will likely have tried a similar ploy on you in the past.
Note - Once you reach the age of 38, Chesnick's rule is reversed: If you are able to date women younger than , you will be admired by your peers.
Another popular method, from the "if you don't get caught, it's not illegal" school of thinking is to be extremely discreet about the encounter - so discreet in fact, that no-one but you knows it ever happened. Such an act led to the age-old philosophical problem of "If he got with a girl of fiveteen, but none of us saw it, did it really happen?" posed by Plato after Aristotle allegedly had a sordid encounter with a young girl in an empty swimming bath.
In a worst-case scenario, in which you are caught, lying about it, or running away are also valid methods of escape.
- see "The Song of Fiveteen" portion of Leviticus in The Bible
- see the "Ode to High school girls" in William Shakespeare's Collected Bawdy Poems
- see any of several hundred rockabilly, blues, country, or rock and roll songs circa 1950 to 2007 with the word "sixteen" in their title
- see Alan Alda's Fiveteen Reconsidered: Old Enough to Stand Up Is Obviously Too Young To Lie Down, But What About Grass On The Playing Field? Oxford University Press, 1977
- results determined from exhaustive tests -- mainly comprised of leering at girls until the girls either giggled or said "Fuck off, grandpa" -- conducted last Saturday at the Hollister store at the shopping mall.