I'm 37 years old now, and I worked at a hobby shop in college in the late-90's early-00's. I can say for sure that we had lots of people my age and younger coming in to buy models. Most did not buy tanks,ships or figures, but airplanes, cars, and sci-fi (Gundams, Warhammer 40K, etc.) were all very popular. There was an especially large group of young people who came in to buy cars and some were really good modellers, too, even getting into scratchbuilding. But what they wanted was different than what I see at IPMS shows even now. The cars weren't muscle cars and NASCAR, but Honda Civics and Nissan Skylines. They would spend a lot of money, too, on upgrades like Dayton-style wheels, "hydraulic" kits, photo-etch, etc. Having grown up with video games and being part of the "younger" generation, I can say for sure the interest is out there.
What they did not have was an interest in being part of IPMS. I was always the youngest modeller at the local IPMS club back then. Occasionally, someone would show up at a meeting, but I think the focus of IPMS was not to their liking. Some might call it critical, but I think most IPMS members enjoy research and history as much as model building. From my experiences in my teens and twenties, the interest was in building "cool" subjects along with your friends. The younger folks probably don't care about being super accurate or detailed, they just want to build models with their friends and socialize. Sitting around and talking about history and molding quality with people 20 or 30 years older than you just wasn't appealing to most teens and twenty-somethings. Just look at the Games Workshop/Warhammer scene to know what I mean. There's plenty of modelling going on there from all ages, and good stuff too, but it's very social. I was just an oddball who happened to love both history and the competitive aspect of model competitions.
More than likely, IPMS will die a slow death, but modelling will stick around in one form or another. If the IPMS wants to survive, it will need to change - a lot. From what I have seen as a "background member" since around '97/'98, IPMS/USA seems very change adverse. Small changes, sure, but culture shift and change seem anathema. If IPMS wants to grow, it's culture must change. It's got to get more social. It's got to be more fun. Contests are great, and improving modelling skills is awesome, but not much about an IPMS contest, local or national, really helps develop skills or create fun if you're not already stuck in.
Make it fun. Make it social. Create real ways to help people get more out of scale modelling without seeming overcritical. Do that, and IPMS/USA will grow regardless of what age group you focus on. Otherwise, I don't see much value in IPMS for the under 40 set unless you're an oddball like me.