I'm 37. I've been a commuter off and on for sixteen years. Been into C&V for six years. Three of my four bikes are vintage, fourth is a Madsen cargo bike. All of them except the Madsen and my 1992 Fuji Sundance are built from stuff I had laying around or had been hanging onto for some reason or another. When I lived in PDX I dabbled in buying cheap quality vintage frames, refurbing them and selling. I sold maybe six or seven this way and each buyer was in their twenties, presumably college students. Riders under 40 are certainly out there, but those that appreciate C&V seem to be very rare. The only folks my age that are into C&V that I know are bike shop employees.
I first got into it because they were inexpensive and I wanted to figure out how they were put together. Then I got hooked on frame materials and geometries (which I'm still not sure I fully comprehend). Now I love the history of them (like my 1966 Schwinn Super Sport) and that if you know what you're looking at, can get a frame that when new was very expensive and well made. What was once a hobby is now my lifestyle. I commute almost daily on either my Super Sport or my Fuji, sometimes opting for the Grand Jubile but I usually reserve that for more spirited rides where I carry less stuff.
Glad to know there's other under 40 C&V riders out there. Cheers!