I was reading about it on another forum and it seems that the problem is not they're eligible, but it they shouldn't be eligible because the younger they are, they are more flexible/nimble.
So basically, while a properly grown adult would need immense talent and skill to pull off some off their flips, twists etc, being younger/smaller would give them a massive advantage as they could just gain more airtime and rotate faster. Hence the underage rule.
That's bullshit, I know the rule states 16 and that's fair they did cheat. But the rule is total bullshit, you have to give an athlete their due if they are good enough, they ARE good enough, China did win the gold after all. Also, if that was true we would see 12 year olds winning nationals in other countries I dont see that really happening, tbf I dont watch gymnastics ever but I really dont have a problem with what China did there. To say someone cant compete because they aren't old enough is just stupid