purely and simply an alternative to "fuck". the term most likely came out of the austin, tx area in the mid 90's when austin uber alles experienced its new sexual revolution and casual sex became commonplace. fifteen, then, probably refers to the amount of time a good casual fuck takes- 15 minutes. perhaps the reference is to amount of positions.

an age that suks sixteens the year we all wanna be
duh
fifteen suks

v. the act of coitus (slang)

Another, lesser name, for rapper 50 Cent.

When a retard comes up with a retarded naming idea for a concept and thinks it is brilliant.

To randomly add swearwords to an anime-dub to get a higher rating and attract more buyers. Practiced frequently by Manga Entertainment in the 90's.

Fifteen is the age in a boy's life where everything sucks. Period.
Your body turns against you in every way possible. Pimples, voice cracks, terrible body odor, unnecessary hair, constant horniness, and greasy skin and hair. Hormones run wild throughout your body producing an array of emotions you can't even understand let alone deal with.
Meanwhile, you have to deal with high school. Projects, homework, hard subjects, sadistic teachers, social crap to sort out, finding allies; they are very hard and common things there.
Also, all the girls you meet at school seem to flourishing into beautiful women. Their wonderful breasts jiggle and taunt you, although you can't have them. The mean ones make fun of you, the nice ones try to humor you, and if you find an absolutely great one that's willing to be your gal, good luck getting the money to pay for her. Businesses won't hire you for a year or so even if you did have the time to work a shitty job.
And there is no relief from this hell that is your life. Doing drugs or alcohol is a temporary release, if you can find, pay for, and not get caught with it. You're just barely smart enough not to commit suicide or run away from home. And you know that porn you're always watching will eventually get you in trouble. All you can do is wait it out.
Also, 15 is the fifteenth number from zero

A fifteen out of 10, much hotter than a regular 10 out of 10.

"Fifteeens" refer to 15 inch subwoofers in a car audio system.

For those who don't know, that's fuckin huge. And the shit kicks.