The government had chosen them because the underage gymnasts were natural prodigies and China wanted to WIN!
I guess the 16 year olds were not as good as the younger ones and weren't as capable as the younger ones also.
The younger ones were more flexible, more able, and more skilled than the 16 year olds, and were probably much lighter than them too.
But in the end, all these gymnasts turn out broken in the end of their lives. with back problems, feet problems, and much more. They also will not see the real world and have been growing up with one thing (living, breathing, and eating it): Gymnastics.
So please cut them some slack!