The Happy Australian

From a recent paper by Angus Deaton we get this snapshot of reported happiness across age groups.

The results for New Zealand are similar to the other wealthy OECD papers. We see a U-shaped trajectory: the young and those of retirement age are happier than us middle-aged folk. It's easy enough to think of explanations for this, but what I don't get is why the trend isn't apparent in Australia. Australians appear to stay about as happy as they ever were throughout their lives.

Why??