For what it's worth, lateralus, I think you're in my top three favourite people on Ernya. You're pretty awesome actually, if you need to hear it. And if it helps at all, we're not ALL teenagers- I'm in my twenties too, there seems like there's a few of us in that age range here.
But those facts aside, I do know the feeling you're talking about. I'm a very active member of a writer's forum on another site, and as someone who is single-mindedly concerned with novel writing as a career, I take it very seriously. And sometimes it feels like I send my entire damn day giving writing advice to 13 year olds who don't actually give a damn about strengthening their writing skills, because they just want to go back to writing about Harry kissing Draco in Snape's office. Or when I spend half an hour writing up a two-page critique for some kid, who responds with "I don't care you don't have to like it I didn't post it here for your approval I never asked for your opinion anyway." Wtf. And yeah, it gets me down sometimes, so I'll go through a period of time (I'm on one right now) where I'll just avoid that place and pop in every couple of days to see if anyone I recognize as quality writer's-discussion people posted anything I can actually talk about without feeling like a pouf.
My prescription: Surround yourself with the people who make you feel good, or the people you know understand where you're coming from, and then when you get down on this whole internet thing you have a group of friends to bring your spirits back up.