Tell that to Max Michel and Eric Grauffel. They both started at 7. Max was winning local matches at 10 - 11.
It all depends upon the kid. Mine has been shooting for 3 years, handguns for 1. He is shooting local matches with me with a 22 with a dot. He is beating a few shooters here and there as well. If the kid wants to, and can be safe, let them shoot. (Assuming it is ok with your club members).
To say that 9 is too young is to throw way too big of a blanket statement at such a subjective subject. It's not about age. There are 60 year olds in our club who shouldn't be shooting.
It's not about shooting skill, it's about judgment. While I would agree that every kid develops differently (and some never do, like those 60 year olds at your club), 9 year olds simply aren't ready to handle handguns out of the immediate reach of a supervising adult. I believe it is irresponsible and negligent to allow a child that young to shoot USPSA/IPSC, and I believe I have the experience in both shooting and parenting to make such a statement.
The problem is that parents' judgment can be clouded by their eagerness to have their kids involved in their favorite activity. Understandable. And unfortunately, other shooters won't always speak up.
The consequences of being wrong are too severe.