It is illogical for us to insist that a child of a certain age needs to, and is capable of, learning only a set number of items and in a certain order. Instead of trying to force all chilren to progress at the same speed we should be allowing those to can work ahead to do so at their own pace.
We need curriculum that includes progressive acquisition of skills and knowlege but this should not be linked solely to the age of the child. If the child can show mastery in a subject he/she should be allowed to progress in the curriculum at a pace comfortable to them.
Grouping children by age instead of ability creates an unnatural environment which is often stifling to those capable of working ahead.