In China 0.9% of COVID-19 cases were in people <10. In the Chinese population 11.9% of people are <10.

In Italy its 0.5% and 8.4%.

In South Korea it's 1% and 8.4%.

10-19 year olds are also very underrepresented although less severely so.

This seems odd.

I've seen suggestions that this is down to younger people not being symptomatic and hence being less likely to be tested.

Contra this, these countries have implemented ALOT of tests (China: unknown but apparently can do >1.5M/week, S. Korea: 275k, Italy: 125k). I can imagine children being tested less, but this much less? If there are 97 negative tests in 100 positive tests (as in South Korea), I'm struggling to believe that there are hundreds of children out there who are asymptomatic COVID-19 carriers whom the testing agencies are just missing.

On Diamond Princess, with 100% testing, 33% of <20s who contracted Coronavirus were symptomatic, albeit with a small sample (2/6, in under 10s it was 0/1) which isn't hugely far off the average of 49%. 20-29 year olds had a larger sample and symptomatics were 89%. It would be strange to see such a dramatic change and I am no less confused than before.

So, what gives?