There are two different argument going on here.
One is about the validity of the rule. The rule may or may not be valid and that, of course, is a point up for debate. Indeed, any rule in any sport is a point that can be fairly brought up for debate.
If you disagree with a current rule, that doesn't change the fact that the rule currently is in play. We're all sports fans here and can probably all agree on a dozen rules that are moronic. I'll cite my continuing disgust with the recent Michelle Wie "crossed the magic pixie line and got disqualified" event. I don't dispute that the LPGA was right to disqualify her, I just object to the way the LPGA handled it. And I think the rule is stupid.
If one disagrees with a rule, one can work to change that rule. In the meantime, the rule remains in play and enforceable.
The second issue is whether rule regarding minimal age limit of the gymnasts was broken or not.
Even if one disagrees with the rule, if the rule was broken, there should be consequences. The "everyone else breaks the rule so it should matter if X breaks the rule" argument is absurd. If you choose to break a rule, you do so with the knowledge that you may get caught and may be punished for it. The validity of the rule is not at issue - your compliance with the rule absolutely is.
The evidence against China is circumstantial, but significant. Hopefully, the investigation will, at the very least, come up with some convincing explanations for the age discrepancies that can put this issue to rest. It is damaging for the sport for an allegation of cheating to fester for too long.