Definitely the former, though I think it isn't simply that younger people are more inclined to use internet forums. For many here, in particular those still living with parents, I suspect this is one of the only places they have to talk about their issues. They may not be able to talk to their parents about it, and if you haven't "come out" about it to your parents, you almost certainly aren't seeing a therapist, either. Friends, if you even have them, may not understand or you may be too afraid of alienating them by mentioning it. At that point you just want to know that you're not the only one, whereas the older you get, the more that doing something about it becomes important, and the more likely you are to have people closer to you to talk about it with.
You don't grow out of it naturally, of that I am certain. Only though your own efforts will you make any real progress. It's true that over time, you may be forced into uncomfortable situations and be forced to adapt, but your anxiety will always make you drift towards a comfortable shell of avoidance. How much this affects your life depends on what situations in particular your anxiety manifests in. Some could probably make it through their entire lives avoiding those situations, though that's still a terrible way to live.
I first realized I had a problem when I was around 14, and I'm 25 now. As proud as I am at the progress I've made, I haven't even remotely reached where I thought I would be. I had always trusted that someday it would just be over. That day never came. It was only fairly recently that I realized that I was simply telling myself the most comforting lie, the one that meant I could just go on living the way I was without having to face the ordeal of change.
I think that anybody who has found themselves growing older but not really growing wishes they could send a message to their past self, like "You do not have all the time in the world." It will have to suffice to say it to your current self every day.
My two cents, anyway.