New slang pops up all the time, and many of the words that become instant sensations can suddenly go out of fashion as quickly as they came in... so this leaves you with 3 options.1. Use slang that would fit with any demographic.
For instance a world like "Cool" is timeless. It's been around since the late 40's and everyone from a grandma in Milwaukee to 6 year old kid in Tampa knows what it means. It's also universal, watch some foreign language films and you'll see it pop up every now and then. So a word like "cool" is a much better word to put into your work than a word like "epic." And no kid or teen is gonna think less of your story if you use "Cool" or another word that fits all demographics ("Awesome") instead of some newer slang term.
So it's better to stick with the timeless slang for the above reason and also if b/c if you should choose to use a word that goes out of fashion it will make your story seem dated.
And when I say timeless btw, I don't even mean slang that's been around for 5 years, the minimum should be a decade. Look back on the 90's for example and wonder about where "As if," "___'s so Fly," and "Wicked" went off too. All those were used for more than a few years but are nevertheless now extinct, and anytime you see/hear those words you know the book you're reading or film you're watching is old.
2. Forget about wide usage and just use what you want
On the other hand you can totally decide that you don't want timeless and universally used slang. Harry Potter for instance is full of British slang and euphemisms that aren't used in America and Canada, etc and people still read it anyways
you can see a list here http://harrypotter.wikia.com/wiki/British_terms
That list doesn't even include some other words and phrases like "summat,"(something) and "innit" (isn't it) used by some of the side characters. Or phrases like "Taking the Mickey."
3. Make up your own slang
Another option would be is to completely make up you're own slang, which can go either way in terms of reception.
For example Diablo Cody wrote and won an Oscar for "Juno" in which she made up a lot witty lines and catch phrases/words that was supposed to be teen slang, and it was all well received, and many people even started quoting the film. She tried to repeat this in her second film "Jennifer's body," but what felt natural in "Juno" didn't feel natural in "Jennifer's Body" at all. In fact a lot of people in all age groups were annoyed by the made up teen slang.
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Lastly, as far as words like "Funky" go, it has to be believable that the character you're writing that line for would say it. Just like we generally wouldn't expect an older character to say "Epic Fail," we wouldn't normally expect a younger character to say "Funk/Funky" either unless they were talking about something like a bad smell.
There are always exceptions however, you could have a younger character who's m.o. is to use different slang than other characters in his or her age group. (e.g. When I HS i had a friend who often used the words like "nifty," even though no one else did.)
And of course any character could use any slang word in a humorous way, a 45 year old man could make fun of a kid by imitating "EPIC" and a class clown teenager could use words like "bodacious"
[This message has been edited by revilo87 (edited April 27, 2011).]