I don't have the numbers for Australia/other western countries, but there are around 50,000 Chinese juveniles each year, to simplify the estimation, assuming birthday and when they commit their crimes are all evenly distributed during the year and there is no link between birthday and when/whether someone is going to be a criminal, you are looking at a few such cases a year in a country with 1.4 billion population.

the combined population of US/EU/AU/CA/NZ is close to the Chinese population, assuming the crime rate is the same (which is obviously not, thanks to the crime rate in US), you are look at a few such cases each year for the entire western world.

for australia, there were probably a few such cases after WWII.