Here is the link to the thread sbf mentioned. It is actually Australian Census data only; no NZ data included.
Here is the equivalent graph I prepared, for JWs in Australia, based on census data. More detail in the above thread.
Like Japan, it is top heavy (ie has too many old people), which indicates population decline in the future. Unlike Japan (or any other country that I know of), the tree is leaning over considerably; there are significantly more women than men.
By generational descriptions, baby boomer women are the biggest group, and the next biggest group are gen-x women. If you look back at the thread, you will see that there aren't that many women currently in the Borg that are below 40, which will inevitably reduce the numbers born into the Borg in the future. The thread also shows this is a relatively recent phenomenon that has crept into JW demographics. Further, I explain that the data matches the limited data from the US Pew report, which suggests a similar scenario is playing out in USA, and probably most Western countries.