Bless you shepherless - this is fascinating.
Another measure of average is the mode. And looking at your data it looks like the average (mode) member of JWs in 2006 was a 50 year old woman, whereas in 2016 it was a 60 year old woman. This absolutely seems true in the UK. At assemblies 60 year old women are everywhere. There are tons of them.
The discrepancy between male and female infants (0-4) just looks too large to make rational sense. If I had access to the data I'd be checking and double and triple checking for a mistake. This is the sort of statistical result that doesn't pass the "smell test" which is a proper part of any statistics analysis. It just looks wrong and I can't think of any good explanation for it.
To clarify, when you talk about immigration, are you talking about net immigration? Because you cite the immigration figure and the increase, but don't mention any emigration figure.
In Scotland I would say that, of the 1000 or so Adventists, 800 or more are immigrants from outside the United Kingdom. Mostly from Zimbabwe, Zambia, other African countries, Caribbean, Philippines, Russia, other Eastern Europe, in that rough order. If Australia is in any way similar then I think it would be very difficult to analyse SDA trends over time, in this particular format, or compare them with other groups. I'd rather choose a group like Mormons, Baptists, or Methodists, to compare with JWs. But maybe I reveal,my ignorance, without checking I don't know if Australia has many Baptists or Methodists.
I'd also suggest that if you want to chart the trend of fertile females over time, it may be as well to exclude females under 14 as well as females over 40. I guess you could argue that females under 14 will be fertile in the future. But they are not yet, generally. And they may not be JWs by the time they are fertile. I doubt their exclusion would have a huge impact on the trend, but it may have some small effect.
You are absolutely correct that POMI is a crucial target for activism (for those so minded) your evidence, everyday experience, and common sense confirm this observation.
I find these developments absolutely fascinating. I'm just a bit disappointed that I suspect Watchtower will soon stop publishing their statistics in any format that allows their decline to be easily charted.
But there will be census data from some countries such as Australia!