I definitely notice myself being in a minority (being in my 30s). Todays young railway enthusiasts also have the draw of Train Simulator (and whatever else is knocking around these days), in which it is possible to build much larger "layouts" than the average suburban bedroom can hold. I wonder how much of a drain on actual modellers that new rail-related hobby is. Personally I tried Microsoft Train Simulator about 15 years ago, and it bored the pants off me, but I'm just one person. And it did open my eyes to North American railroads.
Saying that, the club I've been getting involved in has a fair few people involved who are reasonably young, at least in the railway modelling sphere (i.e. 20s, 30s & 40s), at least in the diesel & electric group. I haven't met the kettle brigade yet, so I don't know about them.