I'm rather fortunate-- in a strange sense of the word-- that I right secondary-world fantasy, and thus I get to use my own phrasing and slang that is world-specific. There's no need to be current or modern in a way that might date my work faster than I would hope.
I have friends who've written things set in "now", and because of the time it takes to get something sold or publish, the "now" has drifted away from them to a point where their story feels like a relic. Or-- a real danger when writing near-future SF-- when I read Snow Crash, it was already the same year of the "future" of the book.
I have been accused of using language that feels too "modern" for a fantasy novel-- though I think that comes from the strange expectation that fantasy needs to use some sort of faux-archaic tone, which I do not agree with. Now, this might cause my books to get dated sooner than I would hope. We'll have to see.
But the question at hand is also: can you write in a way that speaks to younger audiences without making your work seem dated-- or worse, like your some out-of-touch fogey trying to hard to "relate" with these kids today? I think you can if it's authentic. If it comes off as pandering-- like you're putting on a voice to target the youth market-- then they'll know. And they won't like it.
Now, I've got plenty of work to do today in the word mines (and up here in the real world), so I'm off to it. And if you're attending the Writers' League of Texas conference this weekend, look me up. I'll be the one in the vest.