Gymnasts getting younger (at least for women) has been happening for 20+ years. Older gymnasts aren't as flexible, and you see a lot of modern gymnasts retiring in mid 20s max. A bit ....ed up, IMO, but there you go.
Men are a different matter as there's a lot more strength required for their disciplines
And what are China's so good? Well, in states like that (and the old Soviet bloc) good ones are spotted early and are then trained intensely, by the state because international success is seen as benefitting the state itself..
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