I actually block YouTube on my son's computer. Just a personal choice - not making a judgment on anyone else's choice. Because of the crowdsourced viral content, I don't like being dependent on YouTube to remove offending content, so it's easier to block it and let him view it when I log my password and can see what he's viewing. I have Hulu, Netflix, etc. blocked as well so that we can discuss what he wants to watch and whether his homework, etc is finished before he gets bonafide vegetable time.

My son spends the majority of his time on Garageband and Scratch.

And, you know, it's really about personal parenting style. For us, computer time is a part of the family rhythm and doesn't replace sports, music, or family time. For others, screen time is a disruption. Our kids are gifted, so not being exposed to a computer in the early years isn't going to necessarily mean they're behind on skill sets when they enter school. By the same token, I'm pretty sure my kids weren't damaged from their time on the computer when they were little - and I'm pretty sure the break of them exploring and learning on their own was a massive gift to me. With three gifted kids, their drive and inquisitiveness completely exhausted me some days. When they could play games like Nancy Drew, etc., and have fun together, it was a gift to all of us.