I personally experienced that phenomena. I was not held back but was almost a year older( due to my birth date) than  most other kids in my class. I excelled in every aspect of the school life( I was the class president each year, was first in the class,etc).  My younger friends, however, caught up with me very nicely in later part of high school.  I also remember I was often times bored with the school because I was not adequately challenged.  I was a bright child and I would have wasted less time if I would have started school earlier. Now I've seen many bright children are held back for two years by parents to give them "competitive advantage".  This type of practice may not be advantage at all in the long run.  Independent research in the U.S. on this subject is desired.  What a waste of resources to fund longer schooling if it is not helpful to our youngsters?