My mom is a BIG nagger. She worries about everything. It gets annoying sometimes, but I know she loves me. My dad doesn't really nag, but he's always big on giving warnings on things involving finance, work, and school. You know, kind of pointing out no-nos ahead of time.
I joined the Downs as a teen. I was almost 20, but still, I was a teen, still in school, and living at home- well within my mother and father's nagging jurisdiction, yet I did not receive a single warning or nag about the Downs (except those concerning the percentage of my day I spent on the site, which at times was substantial).
My parents figured that I wasn't stupid and that I could take care of myself. If I would've joined the site and immediately started handing out my full name, SSN, credit card #, and address then something bad might've happened but my parents knew I wouldn't do that (and if I did do it, they would tell me "tough luck, it's your own fault and you know it").
Unless you make bad decisions The Barrow-Downs is very safe. In fact, it's probably safer than going to a mall. You could get into a wreck on the way to the mall. You could get robbed, raped, or abducted in the parking lot. Also, I've been a cashier in the past and if someone came through my line with a store specific credit card I knew a couple ways to really mess their finances up if I wanted to (but don't worry, I doubt the other cashiers ever could think of the method let alone pull it off).
Plus, you'd think a site dedicated to quality literature would have an extremely low number of stalkers when compared to chat rooms, gaming forums, and other places on the web.
When it comes to the amount of time spent on the Downs, obviously work and school should not be kicked to the curb to make room, but if all tasks have been completed and it is time for recreation then why not spend a large amount of time on the Downs? It can be quite beneficial for reasons listed above in other posts. It's certainly better than lots of the things I see kids doing for fun.
Anyway, my parents were always happy to see me doing things in my free time that required thinking and reading. Nobody has anything against those two things, right?
(ironically, after saying school should come first- this post took longer than I thought so I'm late for class now)