There's 10 great ideas, including specific usage examples, in this PDF from Scouting.org entitled "Re-engage with Scouting Alumni." It mentions a lot of ways that younger alumni could reconnect and includes a caveat when a particular idea isn't great for younger alumni.
For councils, there's a "Recommended Council Alumni Relations Committee Structure" PDF, but it doesn't really talk about generational issues when it comes to recruiting alumni. I pored through the Alumni Section of the Scouting.org website, but didn't see much else specifically targeting younger alumni (although the Alumni Adventures sure seems like it'd be a great sell for that audience).
I found one networking group out there for Scouting alumni, the "Young Business Scouting Alumni of Central Texas" group on LinkedIn. This seems like a perfect way to connect to younger alumni while providing a valuable service, professional networking.
San Diego - Imperial Council has a specific division of their Eagle Scout Alumni targeted for 18-30 year olds called the "Eagle Reserve Corps," which provides troop leadership and Trail to Eagle support.
Patriot's Path Council hosts mixers for young Scouting alumni.
Hope this gives you some ideas! :)