Born in 1990, so I'm a Millennial.
I think the big hurdle of getting my generation into modeling is the sheer amount of pass times at our disposal. We can stream any tv show or movie instantaneously. You can create an avatar in a virtual world to live out your fantasies... or even create the virtual world itself. All of your friends are accessible at anytime from a computer you carry in your pocket everyday. I could go on, but don't want to make a giant list.
The thing is, building models is probably as popular with my generation as the one before it, but the subjects have changed and the places they share them aren't on dedicated forums, but social media platforms.
Sci-fi modeling will be more popular with my generation that historical/military. Gundam model kits (Gunpla) are gaining a lot of popularity in the west, mainly amongst millennials. If you were to go on Reddit right now, the Gunpla subreddit is easily the most active of all the modeling subreddits. A large amount of posts are from people just starting out. Star Wars kits are popular, especially Bandai kits. You'll see some Trek and Battlestar in the sci-fi modelers subreddits.
If you want to get a millennial interested in modeling, sci-fi/mecha is probably the way to introduce it. I also think the companies that produce model kits need to look that direction as well. Where are the kits based on popular video game series? Where are the Halo kits? Mass Effect? Fallout? What about the newer TV shows and movies? I still can't believe there aren't model kits for the ships from The Expanse. Give Millennials subjects they're interested in, and you'll get more participants. And once they're hooked on the plastic crack, they'll probably branch out into the glut of armor and aircraft that dominate the market. That's how it worked on me, anyhow.
sincerely,
A lazy millennial who just got off a 12 hour factory shift.