My eight year old just started about a week ago and he seems to enjoy it. He's also pretty hesitant about watching the videos though. My 12 yr old daughter prefers alcumus on the artofproblemsolving website.
It works for me and my oldest daughter and we're 31 and 7 respectively.
Being over 30 myself, I find Khan Academy useful as well.
Even if it's just to quickly brush up on a subject I happen to need and forgot all about.