If the child is smart he/she will still be the top of the class, even if pushed ahead.
Agreed. I have two classmates who skipped a grade into my grade, they both remained at the top of the class. IMO, age and a childs performance in a classroom have nothing to do with each other, in general (That being said, I have seen in a few cases where, in pre school, a child was physically incapable of keeping up with the class. He needed more rest time, took him longer to do things. As he got older, it went away.)