I think it's a few things... first, kids turning 16 nowadays are just plain spoiled. Not to hurt anyone's feelings but if you are 16 and are on this forum with a 9th gen civic that is yours (or your parents' car but you don't share it with someone) and aren't paying for by yourself, you are spoiled.
This theory in plain would generate more forum traffic and make the scene grow, but I think my point is that it's not really 9th gen civics I see 16 year olds with, it's even more expensive cars. In my area i've seen 16-17 year olds driving BMW, Mercedes, etc.
A while back most kids that were getting cars were buying up those older Civics and other similar econoboxes and decided to modify them or had to repair them to get them running well. Now it seems that kids are just handed a brand new car and never have to learn to change their oil because their parents pay for their maintenance at the dealership and/or their car doesn't need much maintenance because it's brand new. Then kids naturally don't get an interest in modding since they've never opened the hood, and if they do gain interest the parents won't let them mod since it's not the kid's car and it's new instead of an old econobox.
The other thing is I see a ton of people, including some members here (don't get offended), paying to have all their mods installed (and I'm talking about easy stuff like base bushings and intakes and even some medium difficulty mods like lowering springs, not tire mounting which requires expensive equipment or tint which you should have done professionally). That's not what being an enthusiast is about. IMO it's kind of poserish to have other people mod your car for you and then drive around acting all cool. It also prevents you from ever learning about your car, which also prevents you from contributing much to the scene. This all goes back to the spoiled kids topic... a lot of their parents just pay for the mods to be installed. Or laziness; I think it varies.
Webby, I think this forum is growing because it's 9thgen specific and it's still a relatively new generation. Same with the 8th gen. But Honda-Tech and other forums that bridge the entire Civic lifetime are dropping in membership and activity because the older Civics that most of those people modded are getting really old now. This is why I am glad I transitioned from 90's Hondas to present day Hondas... everything has already been done to the 90's Civics and it's outright impossible to make your clean looking 90's civic unique in any way. At least with the 9th gen the possibilities are still endless since more products continue to be developed.