Hey Turbo,
We have a 6 year old boy.
We decided at that age that if we decide to let them go online at all, we'll work it backwards from NO website access at all, to what individual domains he is allowed to see. We use macs in the house so it's a little easier to use this approach on OSX. We created a new user acct for him (bare bones restricted user), disallowed access to all settings and apps except Safari.
Then we entered the domains he is allowed to go to: lego.com, pbskids.org (in fact I think that's it). Apple does a good job of blocking in-content ads being fed in, and if he wants to look at something else, he'll come and ask and get told no (most of the time) or we can enable it for a one-time view while we sit with him.
Overall, this has worked very well for us, if at 6 years old he can hack it, I bow to his intellectual superiority and will give him the car keys right away
Now, here comes the 'theory' I have (feel free to skip):
I think everyone here is right, that if a kid wants to view something off limits you can't stop them. They'll crack it or just view it at a friend's house after school when you're not around. This is likely how most kids are exposed to porn the first time anyway....kid finds dad's stash, sharing ensues.
With this in mind, my wife and I have agree that we'll be up-front with our son when he's a bit older, about the fact that we can't watch him 24 hrs a day, and that if he really wants to do something that we disapprove of, he'll likely find a way to do it. With that in mind, if he chooses to do something off-limits, and it comes to our attention, we'll look at the situation one at a time and decide what should be done (if anything, in the case of just plain childish curiosity), and the discipline will match the level of responsibility he had in making his choice, and what the reasons were.
At the end of the day, we can warn him about the dangers of jumping into things before it's appropriate for his age, but will just have to ease him into that self-direction as time goes by.