There is a survey currently ongoing about whether "Youth Group" is biblical. I have often noted than when one is philosophizing one often asks questions about the original question before answering anything, and I feel the need to do so in this case as well. So, I could ask "what do you mean by 'Youth Group'?" but let me be more specific. Why do you suppose the youth should be separated off from the rest of the church like that? I've been in churches where kids going into college played with kids who could not yet talk, and everywhere in between. Like, College aged, high school aged, middle school aged, elementary school aged, not yet school aged, etc., all hanging out together is possible.
I am not saying that there is no room for a particular age group to be particularly discipled as a unit. I am questioning our motives for doing so. If we are doing so because the other ages would find it boring to sit in a room and talk about some of these things, then I at least hope we mean those younger for whom the issues of that age have not quite become problematic in their lives. Even there, however, there is something to be said for being exposed to problems and possible ways of addressing those problems before one is actually needing to live out a way of addressing those problems. Still, it might make sense to treat some topics for particular groups.
My hesitation is primarily that I would expect that intergenerational groups--or even just multiple ages--would be more helpful to all involved. It would also, probably, be less stressful to whoever would be leading a youth group if that person had the backup of the rest of the grown ups, including that old woman who has actually had to deal with this in her children, and then her children dealing with it in their children, or the old guy who has been married for longer than the average youth group leader has been alive, especially if those old folks are still thinking relatively clearly. Sometimes I wonder whether 30-year-olds leading 15-year-olds is a lot more like the blind leading the blind than a lot more often than we sometimes think.
I am admittedly biased in that I have always preferred the company of my elders to the company of my peers, but that does not seem like a bad bias to have.
Yes, there is a generation gap. However, that might be a symptom, rather than a cause, of our youth group mentality. If the generations actually hung out a bit more, maybe they'd figure each other out a bit more, since their worlds would overlap more. Yes, older people have lived through things we who are young have not, but that is just more reason to hang out around them. In hearing their stories, we can pick up some of what they gained in living through those things.
Respect your elders, let the little children come to Him, be one body. Your differences aren't that great, we all need the same Gospel. Get that first, then we can start thinking about what any given group may need drawn out particularly. Seek ye first the Kingdom of God...