Interesting exchange on this thread. I must say the comments of the younger members are quite encouraging in their enthusiasm.
Living in a nation which apparently has the greatest number of railway enthusiasts in the world (Japan- 1 million+ "serious" enthusiasts by some estimates, much than that if casual enthusiasts are included), I will add some observations-
Actually here, there are a growing number of "fans" among both the young as well as a small (but economically significant) number of female fans. One thing is that there never was a "train spotting" emphasis here- the focus being more on photography on the platform ends or lineside. The move away from film photography to digital has actually eased the "barriers to entry" to the hobby by youngsters and women, as decent quality pictures can be taken by relatively cheap cameras that are easy to use. Coupled with the growth of the internet, social media, and youtube, there is much that appeals to newcomers with regards to railways beyond the anorak and spotters book image. There is also much more segmentation in the hobby than in past- some people like photographing trains, others riding them, some recording sounds (traction motors or in-train announcements, etc.), history buffs following old lines and station buildings, mother-young son train watching, train station lunch box fans, etc....
It also helps that the railways here go out of their way to accommodate enthusiasts and hold events (open days, depot tours, etc.) as well as keeping platform areas open and harassment free- when you have so many people among the public liking trains (and people here in general are proud of the railway system and demand quality), being hostile to innocent train watching activity is actually bad customer service. Railways produce goods specifically aimed at railway enthusiasts, for the purpose of PR as well as profit.
Anyway, and some of the younger posters alluded to this, the hobby I think is much more diverse now, and even if there are fewer people on the platform ends , there are likely people pursuing their love of trains in other ways, perhaps less visible.
*to add: some posters mentioned lack of diversity- it helps to have new designs and some quirkiness to attract youngsters and women-the railway scene in Japan is emu dominant, but the sheer number of them as well as the relatively large number of rolling stock manufacturers assures variety and lack of standardization across different railways, though the high replacement rate of the older types has led to some grumbling by older fans (including me, being partial to the sounds of rheostat and chopper control units). However the new IGBT inverter types are popular among the young.