I spent quite a bit of time last summer here on
ZU. And my mom would wonder what I was doing, typing so much. Eventually I just told her that I was on a forum. Well, she isn't exactly familiar with things like that on the Internet, so she didn't take it too well. She probably figured I was discussing stuff with people online and must therefore be a loner or something. But, hey, I was paying for the internet, it was my laptop, and I'm free to use my time as I see fit. She didn't pry into it any further, and I guess she eventually accepted it.
As far as I'm concerned, everyone should have the basic right to use the Internet, and what they choose to do there is up to them. Especially at the age of 18. But of course you've got to consider whether or not fighting with your parents over it is worth it.
Best of luck with it though, and I hope we'll get to keep seeing you around.