Every child is different and each case should be looked at individually.
We were told at the end of last year that since the youngest kid in the class was being left back in kindergarten our daughter was going to be the youngest. Then the school suggested that we leave her back since she would now be the youngest.
As we weighed the pros and cons we were leaning towards keeping her back but there were some “issues” with the incoming class. In the end we had her evaluated professionally and was told not to leave her back. (She scored very high on the IQ test and we were told that if we leave her back she will be very bored in class. She had a very productive year and we are happy with the decision.)