The issue is not the ability to "hold your own" the issue is do you exceed the ability of your peers. These hold backs are looking to stand out amongst the crowd. The playing field isn't how it started out in 2nd and 3rd grade when their skill level and athleticism stood out. Holding their own isn't good enough now that they aren't stand outs they will repeat a grade once sometimes twice to stand out or exceed the skill level of those they will play with as well as against, or that's the expectation anyway. The question is, how do you improve if you aren't being challenged? How is this affecting these boys socially? What message are these parents sending their kids? This has become an epidemic.
No matter how you explain and rationalize the reasoning for leaving your kid behind for them to have an advantage over younger kids you have to wonder about the repercussions socially, and psychologically. We can all read between the lines, your good kid but not good enough, you'll "look better" and "seem smarter" if you go to class with younger kids and if you play amongst the younger kids. How do these kids NOT feel inadequate?