Gifted Issues Discussion homepage Forums Learning Environments Delaying Kindergarten

Here's the most recent link I've seen on this topic.
https://today.duke.edu/2015/03/birthdaysandcrime

Now, this does specifically address drop-out rates, which may not be as much of a concern... or then again, they may be given that boredom can create all sorts of issues.

Another one:
http://www.wcer.wisc.edu/news/coverStories/pros_cons_holding_out.php