Aside from mtb's, cycling demographics almost always skews older.
MTB riders are often from a bmx background, and obviously, there are very few older bmx riders lol. But mtb riding is far more physically demanding, with a ton of climbing and its a great upper body workout. For about a decade or so, suspension didn't exist in the mtb market, so there was that added factor as well.
College racing teams are an exception as well, but I've always been befuddled as to why cycling ranks so low in popularity among sports for 20's and 30's adults. I'm sure cost is a factor to some degree, but if young adults are willing to drop $500 a weekend a few times a year for skiing, is a one time purchase of a $2K road bike all that expensive in comparison? EDC is what, $800 for a single ticket?
I've attended occasional cycling meetings since college and the groups always skew tends an overwhelming majority of seniors.
Cycling is great fun, I just don't know why the sport attracts so few young adults.