My advice is to write in a voice you enjoy and are comfortable with. Absolutely nothing else. Please. If you're inserting modern slang purely in an effort to appeal to your idea of younger readers, then the fact that you dislike it, don't understand it, and perceive such language as degenerated will almost certainly be obvious. Not only will you irritate the younger readers who can perceive that you're trying to fake it without understanding it, but you'll also seriously alienate all those people who take the same attitude towards it that you do.
The fact is, adopting a tone you perceive as "theirs" as opposed to "ours"--setting aside the appalled feeling I get when I realize people actually consider this a "them vs us" situation--would be one of the least helpful things you could possibly do now. It's clear purely from the answers on this thread that there is a massive emotional divide between what everyone sees as the differently labelled "generations". Generation Z are all getting high on iphone usage and dislike reading books? I realize we're all deliberately exaggerating here for humorous effect, but if that's truly our belief, even slightly, there can be no hope of effecting a reconciliation. Surely our goal, in writing, should be in bringing people together over the stories we love, rather than contributing to frustration on both sides. These ideas we have of all the different generations--they are stereotypes. As writers, we should understand the futility of playing to stereotypes more than anyone else. Even if the stereotype is true, mimicry and derision cannot possibly make things any better. Help young people learn to care about good language, don't teach them to mock their own children for their foolishness in twenty years' time.
Because even thoughtless, deluded young people are still, at the end of the day, people. Self-aware. They feel and think much more than we realize, and at a much earlier age than they're able to express it properly. They will be grandparents themselves in half a century or less. Deriding the things they enjoy now, however well deserved that derision might be, is not going to help close the emotional divide between yourself and them (and with such a divide, there are greater things at stake than mere language). Now, writing books about what you enjoy, and expressing it in a way that you enjoy--that will help. The young people who are also open to bridging this gap in communication, they will read it and learn something about the way you think, and hopefully understand you a little better. But trying to offer them the way you believe they think, while feeling only contempt for it--that will surely end in disaster.
Two cents, from a homeschooled young adult who spent most of her childhood reading books from pre-1950 and almost none of it learning modern lingo, or, for that matter, learning to view other generations as something different and other.
