I used to watch this movie all the time when I was younger, and I loved it. Now, I think it seems a little cheesy, but still interesting enough. (Parents aren't going to jump up and down at Sonic's defeat of Dr. Robotnik, but kids might.) There is some violence in the movie, but no injury or blood. Good wins out in the end, and while Sonic clearly has flaws, he is an overall positive character.
I think children twelve or older should see this movie because it is a children's movie, however Sonic sticks up his middle finger at one point and it shows a girl in skimpy clothes. It also has mild language. I'm not sure how bad it is, but I think all they say is Shut Up and Pervert. Anywhere you look it will say the movie is rated G but I think PG13 or higher is suitable.