I've found that pretty much all the holdbacks in our elementary and middle school-aged club program are kids who simply aren't very good or athletic.
You always see a kid, and think- "he looks big for his age," only to realize upon further review he's just a big-framed, less athletic, gangly kid who was held back a year.
He will be able to play maybe a ytear longer than normal, if he was on age. But he will wash out too.
It's a common theme.