Advancing based upon intelligence level means everyone gets a good education. Unfortunately, such a system means classroom sizes will get larger from kids that are held back. However, the playing field is more even when kids are grouped together according to intelligence level. Teaching kids at the same academic level makes teaching them easier as a classroom.
Children develop at different rates. Some children advance quickly while others have delays. All of this should be considered when children attend school. Age is only one factor that should be considered and it should be considered lightly. There are so many adults walking around with diplomas that seem to know so little. It is wrong to pass students just because they have sat through a class.
The students that are held back would be in school longer for every time they are not "smart enough" to take a standardized test. What if my child killed yours? What would you do then? Go jump off a bridge.
Jeez ten more words needed so I have to do stuff. ^.^