Hey Coz. I've got a 12 year old boy with no computer on his own and and a 15 year old Korean exchange student whose parents bought him a laptop before coming to stay with us. My son must either use the wife's laptop or my custom built desktop located in the family room with the TV and stuff.
The 12 year old:
He is not a social kid and doesn't do the social networks and crap. He needs the computer for school and to play games. Still he uses the computer many times without us standing right behind him. He has gone into sites accidentally or based on something a scout or classmate has said that was inappropriate. The first time was an instructive moment and he was pretty upset (thought he would get in big trouble but, for once, he was innocent). However, aside from the obvious protection for inappropriate sites, he also needs to be watched for going into places that will infect the computer with viruses. The windows that pop up and say "a virus was found, click here to clean" are things that he's been told to come get me anytime something like this comes up. He's been VERY good about this and understands the scams now. For added protection I use opendns.com. It's free. You sign up for it and configure your router to route traffic through their filters. You have tons of control on the type of the level of filtering that is to be used but it will affect everyone in the home. I feel pretty safe with this one.
The 15 year old exchange student:
Obviously, since he reads Korean Web sites and opendns.com is based on english text, there's really not much I can do to keep him from going to sites he shouldn't. But quite frankly, I trust him and he's mature enough to self monitor himself. My biggest problem with him is staying up too late. He's often up past midnight sitting in front of his laptop "doing homework". Most of the time I think he's playing online games. I caught him playing one at 2 in the morning. I fixed this by configuring the router to block the internet Sunday nights through Thursday nights from 11:30pm to 6:00am. Since then he goes to bed at a halfway decent time. I allow him to stay up playing games on Fridays and Saturdays.
Hope that helps.